Here is another option to consider. My set up is similar to what you are thinking but different in that I have full queen size bed mattress behind a custom bench seat. My van is the extended version so I was able to the queen bed and bench with a somewhat comfortable "living space" in front of the bench. With swivel front seats it makes a good amount of room. I don't have a pop top but do have a high roof that adds about 15 inches. I have a clear approx. 27" under the bed, about the same above the mattress. We use a futon mattress with a 2" memory foam on top, super comfy. It's at least as comfortable as my bed in my house, seriously.
Here is my thread, go the last page and scan backwards for the most up-to-date photos. http://www.sportsmobileforum.com/for...ead-14849.html
My bench seat is the rear seat out of a suburban/Yukon/denalli, got it off ebay, may have been a 2014. The third row seats from these GM vehicles have integrated seat belts so that may be something to consider too, like this: https://www.ebay.com/itm/Yukon-Tahoe...xXNKxX&vxp=mtr
My seat folds flat so what you could do is mount this seat, install a platform behind and keep your mattress rolled/folded up, fold the seats flat and roll out your bed.
Here is the seat I have: https://www.ebay.com/itm/2007-2016-C...tZbU1H&vxp=mtr
Right now I use the factor shoulder belt on the passenger side, mounted the factor receiver to the floor. It needs repositioned though, crosses the passenger too high. The center position has an integral belt in the bench seat. The drivers side right now is only a ford factor lap belt.
My two kids are car seats still so I'm covered for now. Need to get custom should belts installed for the two outside positions.