Using GVWR to figure a payload is the brightest idea with certain configured trucks from todays new gen high GVWR's with small RAWR numbers.
I see "some" RV owners with 1/2 and 3/4 and one ton SRW report their over the trucks RAWR from using GVWR to determine their trucks payload.
Example is the 3500 SRW Ram Big Horn/LoneStar/Tradesman package with a 12400 GVWR. The truck may gross at 8000 lbs which leaves a 4400 lb payload. The truck has a 7000 RAWR and Ram says it weighs in the 3100 lb range. Add 4400 lb payload and 3100 lbs on the rear axle = 7500 lbs for a 500 lb overload on the rear tires/springs and wheels.
Remember the 4400 lb GVWR payload the truck mfg gives the truck is over the front and rear axles.
The safest and easiest formula for figuring how much payload the rear axle carries from a RV trailers pin/ball load is empty gross axle minus RAWR.
Using the same truck and numbers from above would be 7000 RAWR minus a 3100 lb rear unladin = 3900 lbs max payload.