After rabbiting on in such a disjointed manner, I'll try to sum up.
Bjorn Nyland is right that the MG4 is "practically made for sleeping", with just a few caveats.
- The distance between the tailgate and the backs of the front seats when these seats are fully forward and upright is 1.8 metres or six feet, which will accommodate a standard air mattress (although watch the measurements because some may be longer). The mattress is best placed behind the passenger seat, and it may not be entirely necessary to move the driver's seat forward, although I did that.
- The Trophy's false floor raises the foot end of the mattress to a good sleeping position, but beware the possibility of damaging it if too much weight is put on it. In the SE something else is needed to perform that function; I found that the folding table and chairs, which I was carrying anyway, did the job, so it was actually two birds with one stone.
- The backs of the back seats don't fold completely flat so there is a bit of a slope, but it's not a huge problem.
- Something is needed to fill the rear footwell on the side the mattress is placed, to support the head end. Fortuitously, the plastic storage box (from Ikea) I used to carry the air mattress, pump, sleeping bag, kettle, collapsible bucket and various other bits of kit was exactly the right size, so two birds with one stone again. (In the end I left it there for the duration of the trip, driving with the passenger seat forward.)
- I found the foot end of the mattress tended to slide a bit diagonally, and I intend to put my flight bag in position to prevent it doing that in future.
- The DRLs cannot be switched off while the car is on. This is not a significant drain on the battery but they are bright in an otherwise dark camp site. Covering them with the magnetic blackout curtains reduced the light pollution to acceptable levels.
- I used more blackout curtains to screen the side windows, and they were fine for this. I also used one on the rear windscreen, which was more or less OK, although it hung vertically. I didn't put anything over the front windscreen. I'd have needed two of the curtains, and they'd have had to have gone on the outside. I have ordered a windscreen sun protector from AliExpress which someone recommended in another thread. In black, it will certainly do the job. (Bjorn says that the window covers Tesla sells for the model 3 more or less fit the MG4, so that's another option.)
- It's almost essential to cover the car's screens and the central console, even with the infotainment screen set to dark mode. Another blackout curtain was perfect. (Altogether I had and used eight of these.)
- The key to getting the HVAC to stay on, as noted by Bjorn, is to operate the central locking button on the inside of the driver's door once you're in and settled. If you open one of the doors for any reason, you have to do it again.
- Setting the temperature to about 23°C and the fan to 2 seemed about right to me, though I imagine this will vary with the outside temperature. I didn't use the aircon, but you might have to in hot climates. It's not hard to reach forward and change the settings if you're too warm or too cold.
- In the SR I found that the car used about 1% of the battery per hour, so about 0.5 kWh per hour. (Bjorn estimated 0.7 kwh per hour, but I think he had the temperature higher for most of the night.) As Bjorn warns though, this is likely to be a lot more if it's very cold outside and the heater is having to work harder. Also bear in mind it's not just about sleeping time. If you arrive at the camp site at seven and leave again at nine, and have the HVAC on the entire time, budget for 15%. More if it's cold out.
I charged the car to about 95% on the Fort William superchargers, then drove 14 miles to the camp site, arriving about 7.45. I started to set up immediately (because sunset was at 8 pm), using the VtL to inflate the air mattress. (I didn't have to make a meal as I'd eaten while the car was charging.) After setting up I sat in the driver's seat reading for a while, then went to bed. In the morning I used the VtL again, to deflate the mattress and boil the kettle for both hot water to wash in and to make tea.
I didn't leave the camp site till mid morning, and I don't think I turned the car off at all. (I reset the "from start" trip meter, because it was reading 0 mph and 2.0 miles/kWh and being generally meaningless.) The car was off, however, for about six hours in the afternoon when I went hiking, and left it locked. I should probably make a point of turning off the HVAC from the time I arrive at the camp site until I'm ready to go to bed, to conserve range.
I went back to the same camp site that evening, arriving about seven. I used the VtL to inflate the mattress again, and to boil the kettle for a meal - I had cup-a-soup and tea, but otherwise it was a cold meal. In the morning the car was on 41%, that's with only about 35 miles driving, but two evenings and nights spent in it with the HVAC on. This time the VtL refused to boil the kettle. I have since discovered why - there is a setting in the "discharge" screen limiting the discharge to 50% of the battery. It looks as if it should be possible to alter this, but when I tried it simply wouldn't move in either direction. This is a pain, and I hope someone knows how to change this. (ETA: sorted. You have to tap the setting you want, whereas I was trying to slide it.)
After washing in tepid water and having Irn Bru instead of tea with my breakfast, I packed up and moved on. I went for a 25-mile round-trip drive to the head of the loch before heading back to the superchargers, so all in all I probably drove about 75 miles in total in addition to the two nights in the car. I was on about 20% when I got to the chargers.
So this is all eminently doable, and it's noteworthy that on my first foray into this I felt confident enough to sleep for two consecutive nights without charging in between. The only issue with the car that I couldn't get round was the discharge limit, which surely must be capable of being altered! [Yes it is, silly me.]
I was extremely comfortable all night, and I think the reason I couldn't sleep - at least at first - was that my body felt it had to wake up to shift position during the night. This may be more to do with my sleeping bag than anything else - it's the design that's snug around the feet - and I think it's just a question of getting used to it.
Water can be a slight issue unless you're somewhere with a tap you can use, or right beside a burn or a loch. I only took four litres, which wasn't enough for three days if including washing. However four litres is enough for one day, and if you can just fill up the bottles during the day it's fine.
The main mistake I made was just chucking the food I had with me in a cotton bag, rather than bringing a cool box. The only disaster was the milk, which didn't survive the six hours when the car sat locked in full sun, but it wasn't ideal. I do have a cool box which plugs into the cigarette lighter socket and actively cools the contents, and that's definitely coming along next time. It has a fan which is a bit noisy so I don't imagine I'll be keeping it on all night, and I don't know how much power it will draw, but even running it for relatively short periods should make a big difference. (My main concern was where to put that in the car, but I now see that's not an issue at all.)
The main problem was the midges, which were particularly bad at the time. I had midge repellant and antihistamine cream with me, and even a midgie net, but the net only protects your head, and you can't eat or drink in it, so it's not a lot of use in this situation. No matter what repellant you use you're going to get bitten, and antihistamine cream doesn't really cut it. The Bite-Away wand really does work, so long as you treat each bite quite soon after it happens. I was fine the first night because I did that. Then I lost the wand inside the car, and as a result I'm now nursing a few hundred itchy midgie-bites because I didn't get to them in time.
Moral of the story, be more tidy and methodical with stowage. In particular don't take things you don't need - the wand was lost under the rear headrests I'd stowed in the passenger footwell. Take some sort of insect killer to treat the inside of the car once you've closed all the doors. Research better midge repellants (although I don't think there's anything really great out there).
Another problem would be rain, in that dressing and undressing and washing and cooking and eating have to be done outside the car. Might be worth looking at a lean-to tent just to get a dry area next to the car to do these things in wet weather.
All in all it's very practical and very cheap compared to paying for lodgings. It also saves a lot of time looking for a B&B or a guest house every evening. Having the gear in the car could come in really handy if everything was booked up, and it's a cheap and pleasant way to spend two or three nights away from home in nice weather.