I have been away for a couple of months at a time in a Mazda 121 (which I slept in). The car has front seats that fold back flat, and back seats that do the same thing, giving you quite a bit of comfortable almost flat (the wings on the side of the seat are not flat, but help keep you in place) bed.
Unfortunately the later models don't have this feature. It may be that car safety laws no longer allow new cars to have seats that fold back more than a smidgin.
Last year the Mazda became unreliable, so I got a new car. I'd had it for 15 years, so it gave me good value. Eventually I decided on a 2WD small SUV because I could fold down the back seats and sleep in it (I measured every car), and where I go tends to be dirt roads which need clearance. The sedans I looked at just didn't have the clearance I had in the Mazda (a thing that amazed me - evidently they don't make driveways any more that meet the road at a steep angle). One other problem I found when I was looking was the number of cars that didn't have a real spare wheel. This might not bother you, but here, in Australia, there is often a long way between towns - much more than the space saver spare could be used for. I could (usually) get a real spare, but then the place for the spare would need building up because a real tyre is thicker.
I got three inch thick very dense foam exactly the size I wanted the mattress to be, cut it into three pieces and covered them as cushions. They have worked perfectly to sleep on, but because I am on top of the fold down back seat, I actually have less head room than I had in the Mazda.
Your platform would give much more room. I take some stuff out of the back into the front to set up, so I need the front seats not to go forward. This means I don't need to change the set up to leave where I am camped. I like the fact that if there are porblems where I would need to drive away quickly, I can. I could just hop into the front seat and drive off. It doesn't take more than 10 minutes for me to be ready to sleep and to prepare dinner from the time I arrive anywhere. I suspect that it would take longer with the platform, and that things would be more difficult to get at. I also like to look like an ordinary car during the day, rather than someone who is sleeping in their car.