Lake-
I have seen the XP line up and they are close to what I am looking for but if I am going to spend that kind of money I want it to be exactly what I want.

The idea of having a fixed height rear section allows a few things that I think are important for my build. First is the entry door is a fixed height and at ~5' should make entry/exit easy even if the pop up section is not up. Second is it allows some space/height for moving around inside if the pop up section is not up. Third is it allows for a large rear door that can be opened up for loading bikes and such for weekend trips with the boys. I'm still chipping away at the concept drawings and I'll post some more soon to try to show the main adavantages/features I am going for.