Cargo is a stack of palm trees, 2x4's, or mamma's new washing machine lashed to your vehicle with straps or ropes. If that hides your plate for the duration of the trip, you're excused.
However, vehicles are modified from the way they roll off the factory floor. The manufacturer provides a place 'they' choose to mount a plate. It's not MANDATORY that the plate be in that particular spot on the vehicle. It CAN be moved, if properly lighted, elevated, etc.
Because users often mount tool boxes, ladder racks, topper shells, heavy bumpers, etc. to their vehicles, the law likely views anything 'permanently attached' to the truck as 'part of the truck'. Securing a camper with camper threaded tie-downs causes it to become part of the truck. The 1/2" steel threads on a turnbuckle are no different than the 1/2" steel threads on a hex bolt. Both could be used finger tight, or tightened with a wrench and both provide permanent attachment. And don't bother to argue about 'Fastguns' or such. They can be as permanent as a hex bolt if the user so chooses.
If bolting something to your truck blocks the view of plate, you have to find a VISIBLE place to move it for the benefit of the law.
If you tie down your camper with ratchet straps or a piece of rope, you have an argument.