Most states license trucks based on their "unladen weight" i.e. empty and the Gross Vehicle Weight, GVW.. Many years ago the majority of the states required that the licensed weight be clearly visible on the vehicle.. A half ton pickup generally had "Under 5,000 Lb" stenciled on the door, bed or tailgate.. As the allowed tonage went up the number on the door, etc., increased.. A one tone was 10,000.
When the usage of pickups changed, the majority being used for RV's, the requirement to display the licensed max load was stopped..
Generally speaking a pickup, etc., that is used as an RV can be, and is, licensed as a passenger vehicle, regardless of how heavy the vehicle is..
Commercial vehicles 12,000 and heavier generally must display their weight capacity, via a decal type of sticker on the door. In some cases the maximum weight must be shown in letters 2" high..
When traveling across the country, from state to state, towing trailers and/or carrying a load, even on a pickup, it is a good idea to stop at the first "Port of Entry/Weight Station" that is encountered to check on that states laws..
A friend of mine got in a world of hurt pulling his enclosed car trailer behind his large motor home.. He was over the state limit in the state he had just entered.. He had to hire a truck to take his trailer across the state...