Usually the 12VDC fan has to work to keep the sail fan in the ON position.. IS that happening... This is a safety feature to shut down the propane furnace in the event the DC Fan quits workings... Might need to WD40 the sail fan some... 12VDC DC also runs the thermostat...
I am assuming your propane tank is feeding the main kitchen stove burners OK... All burning nice clear blue flames right... Red Orange color means you have an air leak in propane line... I always ran the burners first before turning on the propane heater to make sure I have propane haha...
My older trailer has the propane tank select switch. I imagine the newer ones is auto select
If both of these are working then it has to be obstruction in the propane line somewhere... Spider webs, rats nest around the sail switch is what alot of folks fine etc... Insects/Wasps crawl in from the hot air cover outside.
I'm sure your trailer also vents heat from the propane furnace towards the water tank... being down in the 20's might freeze that up if the furnace is not working...
just some of my thoughts...
Roy Ken