I think just sticking with and improving a 5th wheel is the way to go. First most 5th wheels are to tall, they go for that stand up bedroom over the truck bed. I think if you just put a crawl up bed over the bed of the pickup and kept the rest of the 5th wheel to a overall max height of 8' off the ground sort of like this:
Then get the A/C in the wall and off the roof and then even better taper the top down as it goes back.