I have the same problem with the Greystone fireplace in my TT. It does't matter what the fireplace thermostat is set to. If yours is like mine the fan is behind the upper front grate close to the thermostat bulb and it seems it deflects the warm air enough that the thermostat bulb gets warm before the room. I have found that leaving the grate open when I use it works best.
Richard & Jill
2014 Flagstaff 832IKBS Classic Super Lite
2018 Silverado 1500 Crew Cab Z71 4WD All Star Edition
Camping since 1989, seasonal since 2000
Car Shredder Op/Tech, Scrap Metal Recycling - retired
I deserve hazard pay when camping with my relatives.