GCWR is commonly found in the vehicle owner's manual, and is read off a chart based on chassis type, engine, transmission, and axle ratio.
It is largely a performance/warranty number, the maximum weight at which the performance of the vehicle will be acceptable to the general public, and the maximum weight at which the manufacturer is willing to warranty the vehicle.
Towing a small aerodynamic car behind a rolling barn like your Winnebago, you will hardly notice a difference from running empty.
It is perfectly legal, at least in the USA. The only law that really applies is the 20,000lb per single axle law, but if you have a 1985 Winnebago loaded to 20,000lbs per axle, it is not going anywhere anyway.
