Well, the more air in the tire the more load it can carry. So the max load is listed on the tire and so is the max pressure. You say the rating is 1950@51psi, well four tires would be 8800lbs. Your Roadmaster is just about 1/2 of that. Even at 30 psi, my the tires on my Roadmaster show wear of over inflation(center wearing faster). My Fleetwood is about 450lbs heavier than the Roadmaster and is speced for 235/70/R15 too, and 30 psi seems about perfect on it. The STS uses 235/50/R17@30 psi and it rides like it is on wooden wheels, horribly harsh ride. Its heavier than the Roadmaster and its tires wear as slightly under inflated. But the ride is so hard I keep it at 28psi, just to be easier on my kidneys. Too bad a 15, or better yet a 14 inch wheel can't fit the STS, a 70 or 75 series tire would ride so much nicer
Those 'Nexens' you are buying/bought are they passenger car tires, or Light truck tires? That might explain the high(51psi) max pressure. When I search online for tires for the Roadmaster and the Fleetwood, Tires for trucks are often in the search results