I think you have the right idea using alufiber or FRP i.e. Filon as an exterior. Stress skin construction and keeping weight down while maintaining integrity.
One possibility for flooring would be to use Compositec
http://www.compositek-inc.com/superdek.htm or something similar, it would reduce weight and not be subject to rot. Given a choice this would be my pick. It was developed by Kaiser for use in aircraft. Forest River is using it in their RV's.
One of the down falls of Eye4Design is that no two Mega-Mini's were the same, great for the customer, but it meant that they never got into a profitable production. I made suggestions to speed up the process i.e. do a wiring loom which can be done on a sheet of plywood, label the terminals... rather than running individual wires. The MM is a fantastic product and cutting edge (except for the floor

) but you have to be able to not lose money!