This s the exact reason the feds stepped in and standardized the weight rating and placarding process for RVs. Nowadays (since 2009 or so) OCCC is defined the same between RVs and the weights must be placarded in a standardized place.
OCCC is a bit different from most (and there are more than one) definitions of CCC.
OCCC is cargo + water (grey, black, potable) + people. The base weight of the vehicle is empty with all added options included in the weight +full LP plus full fuel (for motorhomes). They also are required to state the weight of water so you can make the trades to stay within weight if needed.