I would say, if the motorhome is on the road as a vehicle, then vehicle rules would apply, which according to RCW 9.41.050 is unloaded and not concealed on the person. Any other place in the motorhome is legal as long as it is unloaded.
When the motorhome is parked as a habitation, I would say that the 9.41.050 and 9.41.060 exceptions apply:
RCW 9.41.050
Carrying firearms.
(1)(a) Except in the person's place of abode or fixed place of business, a person shall not carry a pistol concealed on his or her person without a license to carry a concealed pistol.
and
RCW 9.41.060
Exceptions to restrictions on carrying firearms.
The provisions of RCW 9.41.050 shall not apply to:
(8) Any person engaging in a lawful outdoor recreational activity such as hunting, fishing, camping, hiking, or horseback riding, only if, considering all of the attendant circumstances, including but not limited to whether the person has a valid hunting or fishing license, it is reasonable to conclude that the person is participating in lawful outdoor activities or is traveling to or from a legitimate outdoor recreation area;