For what it's worth, I think it would work fine and I'm in the planning stages of building something similar. One thing I'm doing to give a little extra support is to put 4 to 6 steel studs in the walls of the camper. The steel would be welded to the floor and have plywood on the outside and inside of the steel stud. Basically, it would just give a little something extra for the box rigidity during off road trips. A good soft suspension is also a key design tip for taking something like that off road. I've definitely saving that site though, some good tips and construction techniques I've been thinking about lately.
Don
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