2003 Newmar Dutch Star Overheating

My 2003 Newmar Dutch Star suddenly started overheating according to the dash gauge. 

the 1st place I took it said it was the thermostat and replaced it, wrong still registered hot. 2nd place said it was the water pump and replaced it, wrong still registered hot. I found a build up of dirt behind the 2 cooling units in front of the radiator and cleaned it the best I could but depending on the outside air temp, driving speed and/or terrain it still want to run hot.Anyone got any ideas? 

Thanks,

Rob