Thomas has the right idea, imho... these are teardrops; there's never going to be 15 people walking around on the floor. There's no need to support that kind of thing.
My chassis will be a teeny bit simpler than even Thomas's... I'm going to build a 4H X 4 (or 4.5)W X 9L stretched Lil' Bear-style teardrop. The chassis will be one long cross w/ 2 diagonals from just behind the tongue to the axle cross. A 3/4 floor will be bolted to the chassis, then the walls and bulkheads will be dado-cut and bonded to the floor. It will be very strong and quite light.
A "Kit"-style/shape teardrop's chassis is extremely simple too, especially for it's day. People have a tendency to overlook the very long useful lifespan of those types of chassis.