If we were to put this camper on our truck would we overweight or would we be legal? Is there anything that can be done to make our truck legal to haul that weight if it is not?
The short answer is that you can do mods like air bags, extra suspension overload spring bumpers, shocks, 19.5 commercial tires and wheels to better handle the load. Nothing you can do will be able to change the manufacturers designation for gvwr or payload. Some aftermarket body upfitters are allowed by the manufacturer to change the numbers. Since you don't define 'legal'? Then what is
illegal about a pickup carrying a camper that is at or slightly over payload but still under the truck's tire, axle, and wheel maximum ratings? Ask the dealer how exactly
you can do mods to get the weight rating increased by the DOT. Make them put it in writing. Basically you can't! I would move on to another dealer that provides better solid information. The dealer is looking to lose a sale if he/she cannot convince you to put a ten gallon camper on a seven gallon truck.
If you also plan to tow a trailer or are going to worry every time you drive the truck anywhere with the camper get a 450/4500 or 550/5500. There is something to be said for peace of mind. A 6000 pound camper is very heavy.