I had a second generation Grand Caravan. When I took the captains chairs out of the middle, moved the rear seat up there, I could carry five people (including driver) and have enough room behind the seat for two (short) people to sleep on the floor. Or I could carry a lot of stuff. Three 80 foot rolls of rubber dance floor one time.
My other alternative was to just pull out the captains chairs and use the middle as my big empty space. That space was more accessible, but did not have as much length fore-aft, though a bit wider since it didn't have wheel wells. This is the space most often converted to sleeping platorm in budget minivan camper conversions, but it depends on the middle seats being removable to leave a clear flat floor, which is not always the case for modern minivans, depends on brand, model, seating options.
Not sure if any more modern minivans with fold into floor rear seats offer the same options, and you may have too many people in any case. With rear seat folded away for the night, you would have about the same room as earlier vans with rear seat out, or sometimes the seat even folds bed flat, though it is a tiny bed, barely 4x5 feet.
I find a self-inflating sleeping pad worked well enough for me on the van floor, since it was seamless, carpeted, and had only the two sockets for the seat anchors to serve as lumps under the pad.
If you need the rear seat for passengers, you might have diificulty carrying the camping gear you need. At this point some kind of trailer starts to look good, and a teardrop can serve as a bed and a light cargo trailer. An a-frame folder might be configured two beds (four people) an can also carry a light cargo in the little bit of availabe floor space.
I've seen camping conversions on minivan (and smaller similar MPVs) in Europe, and they don't put much emphasis on a "living in it" RV experience. Seating gets sacrificed to a bed platform for two people, there might be a slideout rear kitchen stored underneath. If there is a need to carry 4-5 people, sleeping space ends up outside the vehicle.
You seem to be concerned about overnights on a trip. Even with my motorhome, when making a fast run to a destination, I don't always "camp" every night. That can cost 2-4 hours of driving time, and when I'm really in a hurry my camper gets parked in the parking lot of the motel where I am sleeping, checked in about 10 PM and on the road again before 6 AM. 14-16 driving hours. When RV touring, my days are more like 4-8 hours moving.
There's a compromise in between, and people do sleep in their RVs at rest areas, truck stops, parking lots, or pulled over to the curb on some street. But if you sleeping arrangements include putting up a tent, or even opening a popup or fold out, those options start to be limited.
There's somebody near the coast who designs and builds custom conversions of smaller vehicles like minivans and compact vans or MPVs. I don't have a link, but it shows up near the top of search results for minivan camper conversion.
