There is no normal reason for pressure to build up like that in an RV system. The air "head' in the water heater is normally sufficient to cushion the modest pressure build-up from the heater. Many (but not all) RVs have a check valve on the cold inlet to the tank anyway and this prevents the pressure from pushing back into the rest of the system.
My guess is that the rig you looked at had lost the air cushion in the water heater tank. It is easy to restore by letting the tank cool off, shutting off the external supply (pump or city water), opening a faucet to relive any internal pressure, and then opening the PT relief valve to let water drain out to the level of the PT valve.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL