Your propane heater should keep the inside warm enough as long as you do not run out of propane. They do have plug in 1500 watt electric heaters that can help keep things comfortable and lesson the dependence on the propane furnace.
I would make sure that your fresh water tank is topped off and then put a 100 watt lamp in the wet compartment to keep the water pipes/pump from freezing.
At Home Depot they have rubber insulation that goes over 1/2" plumbing pipes. These usually come in 3' sections and I have seen people use them to block any wind that may come in around your slides. If your slides seal fine, then do not worry about them.