I would look closely at the radiator. If it is original then is it plugged inside? If not, is the fins plugged with junk. If it was replaced is it with the correct one, or a cheaper one with less cooling capacity.
A radiator shop should be able to answer those questions for you and at least rule it out as the cause.
The fan clutch is also worth looking at. Put a piece of tape on the blade and off to a stationary item. When the engine is cold start it up. If the tape is torn then it should be an OK fan.
I had a 460 Class A and had no problems at all like you are. But I changed the fluids, kept the fins clean, and paid attention to make sure the fan clutch kicked in when I figured it should.