I have a long bed Frontier Super Cab with a Leer cab high cap and I'd be willing to cut a hole in the top of the cap to allow access into the RTT from the bed. Haven't measured the top of the cap yet but it's probably close to 50" wide, so a 30" wide bed area 6'-6" long, an open area at the rear where a person could enter the cap from the ground, open the top and have a 20" x 24" area to access the "upstairs" and have another area roughly 20" wide and 4' long under the lifted tent top for whatever you want...to eat, read, cook or...well you decide.
Oh yeah...the top needs to support a 16' canoe, 14' kayak or my 13' Kaboat. I'm thinking 0.060" 6061 on the sides and a frame for the top with some alum channel on top to support boats and maybe 0.035" on the top and sides. Some heavy silnylon and netting for the soft sides.
The idea would be to bolt the RTT to the Leer cap and extend a part over the truck cab but not attached to the truck and make that portion into an aerodynamic shape to reduce drag. Overall depth could probably be 8" or maybe thinner and still have plenty of room for a mattress to remain in place when closed. Of course it would need a roller on the back to slide boats onto the RTT and some tie-down points to handle whatever needed to be hauled on top.
GREAT idea JJ!!