I own commercial trucks and you do not have to stop at any scale with a motorhome unless it is in Commercial use. Says it in Commercial truck Laws.
This is Ca law which is similar to all the other staes.
Every driver of a commercial vehicle shall stop and submit the vehicle to an inspection of the size, weight, equipment, and smoke emissions of the vehicle at any location where members of the California Highway Patrol are conducting tests and inspections of commercial vehicles and when signs are displayed requiring the stop. Every driver who fails or refuses to stop and submit the vehicle to an inspection when signs are displayed requiring that stop is guilty of a misdemeanor.
CVC Section 260 defines "commercial vehicle" (bolding added):
260. (a) A "commercial vehicle" is a motor vehicle of a type required to be registered under this code used or maintained for the transportation of persons for hire, compensation, or profit or designed, used, or maintained primarily for the transportation of property.
(b) Passenger vehicles which are not used for the transportation of persons for hire, compensation, or profit and housecars are not commercial vehicles. ...
(c) Any vanpool vehicle is not a commercial vehicle.
(d) ...