If the thermostat has no air circulation it will not know when to shut off. If the thermostat cannot sense the air temperature while the slide is in then the furnace could run until the propane ran out or the battery went below 10.5 V (assuming you are not plugged in to shore power) or the trailer burned down.
If the thermostat is blocked don't run the furnace.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.