Calculating the weight and balance for RV trailers is a little different because the axles are never going to carry the full weight of the trailer. When setting by itself the unit is kept level - fore and aft - by it’s landing gear. When being moved from place to place some of that landing gear load is carried by the tow vehicle.
Before the RV trailer leaves the factory some of its safety equipment must be certified. That includes its total weight (GVWR) all axle weights (GAWR) and its cargo load capacity. The GVWR is the build target. After the GAWR values are set and a hitch weight has been determined the cargo capacity is determined. By regulations, the total GAWR, when added to the established hitch weight, must equal or exceed the GVWR. Once delivered to the middleman/dealer, adjustments may be required in the cargo capacity if options exceeding xxx weight are installed by the dealer.
The manufacturer’s established hitch weight is an ideal figure. After the vehicle is sold that ideal figure is controlled by the owner, as is all other weights and the balance of the unit.
This is an excerpt from a NHTSA Q&A pdf.:
"The FMVSS have requirements for the manufacturer to use proper tires and rims for the gross axle weight rating (GAWR). The manufacturer may determine the GVWR by adding cargo capacity (if any) to the curb weight of the vehicle as manufactured. The wise consumer, before purchase, will determine if the vehicle has sufficient cargo capacity to carry the weight of water, additional equipment (such as televisions, and microwave ovens), and luggage. The manufacturer’s certification label must show the GVWR. The GVWR must not be exceeded by overloading the vehicle. There is little the government can do to assist a consumer who has purchased a vehicle that has insufficient cargo capacity for its intended use."