The thermostat has to be changed. The thermostats are actually a disc that pops at a preset temp. As you can see in bflinn's post they come as an assy. They are simple to replace.
1998 Pace Arrow 35 ft. F53 Ford V10 2012 Honda Civic toad
32 years mechanic at Delta Air Lines 15 year motorhome service manager. 3 popups....2 travel trailers....5 motorhomes....loved them all.