Just like in ICEVs, there are now many hatchback "SUV"-ish EVs. I have one, I tried camping in it, it's pretty convenient in wet or cold weather. DISCLAIMER: I'm short. Fold down the seats and TEST your car first before trying sleeping in it.
Camp Mode
Teslas have a "camp mode" which shows a really cute video of a fire for a while, during which time it keeps the climate control going. If you don't have on any kind of fan, then you get steamy windows and uncomfortably humid conditions. I've read that other EVs have something similar.
So, to sleep in the back of such a car, you have to throw any of your gear out of the back either around/under/on top of the car, or into the front seats. I have a huge amount of gear, but a lot of it was OK outside, even in wet, so that was no problem.
Other cars, like the Rivian SUV, are likely larger. I wonder if there will be camper shell inserts for EV pickups soon?
Here are issues/observations unique to my experience in a Model Y:
The back is not perfectly flat. It was ok, and I'm super-sensitive to this, but it was not bad at all.
It was winter, so I brought a winter sleeping bag. The coldest you can set the climate in camp mode is 60F. That is just way, way too hot for me to be comfy in my winter bag. So, I had to turn the system to "minimum "temp, where you get some fan but no heating, a couple of times. If you want this to be perfect, you might have to match your sleeping bag weight to the actual temps you can acheive. In the best of all possible worlds, camp mode would allow temps down to 50F, so you could use a midweight bag.
If you keep the climate on, but low temp all night, in around 20F outside, the car uses some 5%-ish during a long winter night.
On occasion, I notice significant drain, up to 10% in one night. On other occasions, I note much less than 1% per night! When I turn off all the phones with the app for my EV, on all these occasions, the drain is very low. I belive that a major drain is the car trying to communicate with the EV app on your phone; this may be paradoxically worse in the backcountry when there is no or limited signal; the car may be trying over and over to try and clarify garbled messages to/from the phones. So, I'm not sure this is the problem, but I have it as best practice to turn off the app and/or phones for your EV whether sleeping in it, or whether you are gone in the backcountry and need to come back to a car without a drained battery. I also don't yet have a good sense just how much energy the entertainment system uses. You wouldn't think it would make significant power drain compared to driving, but in one case of watching movies it might have. Best practice- monitor carefully if you're using the entertainment system for an hour or more.
HINT: Test your fit of pads and people before you go. I have an extremely large, plush sleeping pad ("Megamat"), and you can't put this and any other pad flat in the back at the same time. So, test your pads for fit before you go.
Maybe most EVs are not as comfortable as your friend's tricked-out van, but you can get along even sleeping inside them, and it's even convenient and easy in the wet. Just remember two things: #vanlifenotgreen; those big vans are serious gas-guzzlers. #vanlifenotgreen ! Your friend's "cool" cross-country trip is a small environmental and climate disaster. The second thing to remember is that your friend may not have told you this, but a tricked-out Mercedes van probably costs $150,000-$200,000. I'm not joking, or exaggerating, check this out yourself. So yah, their very non-green toy costs about as much as two of the most expensive EVs you can find. #greenyourride instead!