tow and payload ratings

[jmtandem]

Any advise would be appreciated. I know this is a constant issue with heavy pickup campers and it has probably been answered many times but are the payload or tow ratings (GVWR or the combined truck and fifth wheel or trailer rating) that are provided by the manufacturer based on testing by Dodge, Ford or Chevy/GMC, or are they computer generated numbers that are a best fit all around good rule of thumb value for the strength of the trucks major components? Also, is an overloaded truck illegal or just not recommended based on manufacturers values? I have a 2001 Dodge 2500 quad cab, diesel, 5-speed, 4x4 that weighs about 7000 pounds and has a GVWR of 8800 pounds, not much payload for a camper even without people in the back seats or water in the tanks, yet the camper sales staff insists that with air bags and Rancho shocks going almost 2700 pounds of camper empty weight is not an issue. I have also been looking at toy hauler fifth wheels (30 feet and up like the Patio Hauler) that weigh empty more than the truck is designed to tow. In the case of toy haulers the weight difference is not a few pounds over the tow rating, loaded some weigh as much as 18,000 pounds or about 8000 pounds more than the truck ratings. Heavy fifth wheels are on the highway all the time behind pickups; are there ways to mitigate the weight?
Thanks,