I've been on this forum for 13 years now and not once has anyone ever produced chapter and paragraph from the law books of ANY state in the US where it says exceeding the manufacturer's GVWR on a pickup truck is illegal.
Registered weight is something entirely different. That is what you pay for with the DMV. You can register your truck for anywhere from the base curb weight up to the sum of the FAWR and RAWR in many/most states.
Registered weight is legally binding for COMMERCIAL vehicles, meaning hauling for pay. RVs are not commercial vehicles and so far, a pickup with a camper is universally considered an RV by DOT officials. We have yet to hear of a case where an RV was chased down, pulled over, inspected, and cited for not entering a DOT inspection station.