Does your trailer have a battery onboard? If it does you probably have a 12V charging line to keep the battery charged. Whether or not it would supply truck battery power without the engine running depends on how the truck wiring is configured. Often the truck mfgr installs a device in factory-installed trailer wiring to prevent the trailer from discharging the truck battery. In that case you cannot draw power from the truck battery while the truck is not running.
There is a way to run a jumper wire to accomplish what you want, but there may be unintended side-effects to the truck electronics.
IMO your best option is to install a 2nd trailer battery for overnight furnace use without totally discharging the present trailer battery and prematurely ending its lifespan.
This website may help understand your trailer to truck wiring
although it does not show a trailer battery, which would be connected to the 12V power wire:
2000 Winnebago Ultimate Freedom USQ40JD
, ISC 8.3 Cummins 350, Spartan MM Chassis. USA 1SG, retired;PPA,Good Sam Life member."We the people are the rightful masters of both the Congress and the Courts - not to overthrow the Constitution but to overthrow men who pervert theConstitution. "Abraham Lincoln"