You need to talk to the manufacturer and find out what single part of the entire chassis is the limiting factor, and strengthen that part. Maybe it's the frame itself, maybe it's the hitch, the springs, the shocks, the wheels, tires, or any number of other things that go into the complete package. You find the weak part and strengthen it; then the next weakest part becomes the limiting factor, ad nauseum.
Or you can do what most people do and ignore the limits sets by the manufacturer and just tow what you want.
Alan Hepburn - San Jose, Ca
2007 Bounder 35E being pushed by a 2008 smart fortwo Passion
Good Sam Member #566004 * FMCA Member #F431612