going up a modest hill...
sounds like this rear radiator needs to be cleaned..have you ever done that...when I had a rear radiator Holiday Rambler, did it every year with a hose and Simple Green...If you never did this, then I bet despite the good antifreeze..there is dirt in the fins which on the hill caused the engine temp to rise in a short time...some people downshift to get more speed on the radiator fan....Just a guess, but that's the thing I am leaning towards...and a change of fuel filters at the same time too.