The foam doesn't look like it would absorb water but I'll have to try it sometime. When it cures it gets a smooth skin on the outside that looks like it's water resistant. I haven't had any trouble with water yet.
This camper has an 8-foot floor length so it does stick out past the back of the bed and it lines up with the end of the open tailgate.
I think you (or someone) did express concern about the tailgate window. I hadn't though about it at first but I'm not too worried. The rear suspension rides pretty smoothly with the extra weight on the back, and I'm a gentle driver. If I were rear-ended or backed into something hard enough though, it would probably wreck the camper and break the glass in the tailgate. I guess I'll only worry about that if it happens though.
I originally wanted to find a camper that I could set up more like a Chinook or Chalet, with a pass-through to the cab and camper seating when traveling. I think there are some campers this might work with (maybe Hallmark, with the forward, opposed front benches) but this camper is too low-profile for sitting on the camper bench with the roof down, and the water tank takes up the forward part of the floor.