RV Manufacturer versus Tire Company air pressure settings

The right way is to weigh it and then adjust for the amount of weight, hopefully four corner weights. I go 5 psi over the heavier tire on each axle because I'll drive when there is a big cross wind which shifts a fair amount of weight to one side.
Unless you always travel without any water in any tank (which I doubt) then maybe. I weigh mine with full fuel and water plus all our normal stuff on board.