In CA, pickups are defined and registered as commercial vehicles. You can register one as an RV, but can never carry ANYTHING in the bed after that.
Also in CA, the DMV defines a truck camper as a "pickup with a load". However the health department classifies it as an RV because it has cooking and sleeping facilities.
Finally in CA, the trucks have outdated the laws. A pickup is defined as 11,000 GVW or less. Most duallies are now 14,000 GVW, and are therefore not pickups under the law, must have a weight sticker on the door and stop at weigh stations. Practically speaking none do and I believe you would be waved on. But I haven't tested that.
When I bought my F350 the dealer registered it as a 14,000 lb commercial vehicle. If you look at the DMV registration categories, it really doesn't fit any of them, dating from before 14K pickups. It took me several months and about 6 hours at the DMV to get them to change the registration category to "pickup". I think technically this downgrades it to 11,000 GVW, introducing the risk that I could be ticketed, fined, and taxed for being over (about 13,200 w/camper).
Bypassing a weigh station that you are legally required to stop in (in CA) isn't just and infraction, it is a criminal offense - a misdemeanor - if they want to get serious.