You can register a pickup as an RV under certain circumstances, such as for the purpose of pulling a 5th wheel RV. Also if a truck camper is "permanently attached". You cannot use a pickup registered as an RV to haul goods, commercially or privately. By strict reading of the law, you therefore cannot use a truck registered as an RV to haul a slide in camper that is not permanently attached, as it is defined specifically in the CVC as a load. Since it is legal to pull a bulldozer on a trailer with an RV, maybe that's ok, haven't researched that one. Enforcement may be something entirely different, and may change from time to time or place to place.
The 14,000 lb question is a good one. Technically none of these new big pickups are pickups in California. If given a ticket, I would argue legislative intent: they clearly meant to exclude what we think of as pickups when they passed the law, pickups now have gotten fatter since but are still within the intent.