The truck engine is idling during the time you need to be in the trailer, and the trailer has no battery but does have a working converter I assume. Also no portable generator handy.
Need for heat and doing some work in the trailer suggests this could be for a longer time than just a few minutes.
So another way is to attach an inverter to the truck's battery and plug the trailer's shore power cord via as long a 120v extension cord as you need into the inverter. The truck engine still has to be idling. The hood needs to be up with the inverter hanging out on the fender to prevent the inverter overheating from engine heat and shutting down.
If the power requirement is not so large for 120 or 12 it needs the truck's power, then one of those power box combo inverter/battery things would let you plug the shore power cable into that, then recharge the power box in the stick house.
