I have a 23' Damon mh its old but taken care of 1995. The furnace has worked every winter as I fish hot water discharge lakes in Illinois all year. this year the fan came on when I turned it on but no heat. I filled the self contained lp tank ran it thru whole camper stove, water heater to get air out. I checked for obstructions via the exhaust and plenum. seems like no ignition. checked fuses I feel like ( I know nuttin) Its the thermostat or ignitor but don't know how to test either one Help please