[JRTJH]

Any space heater you use will add about 5000 BTU to the inside of the RV. None of them will heat the "basement area" or the "insulated underbelly area" so you still need to run the furnace to keep that little bit of heat blowing into the area down there. As for skirting, if nothing else, get some 6 mil plastic or go to Harbor Freight and get some of their blue tarps, and attach them to the RV and cover the base around the ground with snow/2x4's or some other anchoring system. That is not the best, but at least it will block the wind from getting under your RV. Ideally, some 2" foam sheeting (R-10) or some OSB sheeting would be better. AVOID anything like bales of hay, etc !!!!! All that will do is attract mice, rats and other pests and is a definite fire hazard as well as a great place for mold to grow.

Once you do get the skirting installed, adding a 60 watt incandescent bulb will do much to help provide that little extra bump to keep it warmer under there.

It's almost Jaunary, so you're already in the coldest part of the year, trying to improve on what you have already done pretty much means working outside in the cold, on the ground and under your RV. That's certainly not a pleasant thing to endure. So, be sure to protect yourself from frostbite, cold winds and the wet clothing that you'll surely encounter.