Like yours, our trailer is a 3 season without a enclosed or heated under belly. We regularly stay where it drops into the 20s at night and are fine as long as it gets above freezing the next day.
To keep us warm we have used both a small electric heater when we had electric and the furnace while we boondock, both do fine.
Things you may do if you wish...
add some anti freeze to the waste tanks.
Remove the city water hook up over night or use the FW tank.
Open the cabinets and drawers where pluming is so the heat gets to them.
We put heavy bath towels over the windows to cut the heat loss. Rugs on the floors help too.
If you are going with a electric heater set the furnace to its lowest level just in case the electric fails somehow.
