Top left of the thermostat is the on/off button. Push (maybe hold) to turn it on. The indicator will light up.
Push the "fan" button until it reads "auto".
Push the "mode" button until it reads "furnace".
Push the up or down arrows until it reads the temperature you want. (You can select F or C with another button.)
If the temperature in the trailer is cooler than the temperature you selected, the furnace fan should come on right away and the igniter will try to light the furnace.
If it worked last night, should work again unless you are out of propane. Both propane valves must be open for the auto regulator to switch from one to the other automatically.
Or unless something is restricting the outside furnace exhaust, such as wasps nest.
Other faults also possible. Let us know.