cummins 325 in 2001 HR Endeavor motorhome overheating

Has anybody had this problem and how did you fix it? On long hills, in hot weather,(80's) our motorhome heats up - example the long hill out of Palm Springs going east on I-17. We have had radiator guy check rad with heat gun and he felt the circulation was OK. Had had it washed out with a power washer prior to that. Then he changed the thermostat and the transmitter to the dash. Then had gunk remover sprayed on rad and flushed with hose. BUT - still overheating.Going to just over 200 degrees. Any ideas?

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You probably need to downshift before you begin the ascent and keep the rpm around 2,000. You need to keep the fan speed high enough to cool down the engine. I have the same problem going up the grapevine when I am towing my toad.