There isn't much gained on that truck. But I checked mine yesterday and there is 12" between the top of the wheel well and the bottom of the bed. If you are going to leave the camper on all the time you could design the camper bottom to mount to the frame and save weight and cost.A sunken flatbed would be more ideal, like you're saying.
I realize that you can get some of that space back with external boxes but the center part I mentioned does not have easy access. You could in theory stick batteries or a water tank or something under there, but I'd rather have stuff where it's easy to get to. And it's nice to have the entry and main floor ~12" closer to the ground.
IME the time when heat would be nicest is at night *before* going to sleep, when you are doing things and can't be buried in a bag. Or in the morning when you get up and it's cold. When sleeping you should have a really good bag, or like I said earlier maybe you could rig up an electric blanket inside if you have decent solar capacity. A well insulated camper can make a significant difference also.I dunno...since temps drop at night, that is when you need the heater...otherwise what is the point?