Your biggest problem will be moisture build up from many bodies, wet clothes and the stove if it is used. Propane flame from the stove gives off a lot of moisture. The furnace will warm up the trailer just fine but will cycle a lot, less the more bodies there are.
The only way I have found to deal with the moisture is crack open a window opposite end of your unit from an exhaust fan to get airflow through the unit. This can be adjusted as needed. Before I lockup I wipe down the windows and widow frames, open the window wider and turn the exhaust fan up higher. I have run the fan upto 20 minutes before locking and works good. Orient your unit so that any available sunlight will come through windows and even with the unit sealed up there are enough air leaks that over a few days your unit will be dryout.