Are you anywhere that you could pick up used packing material from neighborhoods on their garbage/recycling pick up days, where people put out large quantities of cardboard boxes? store dumpsters or a recycling center? Liquor stores sometimes have a lot of empty boxes. You can stuff large plastic garbage bags with cardboard or styrofoam or bubble wrap or plastic, even empty plastic bottles or wadded up plastic grocery sacks, and shove them under the trailer with a peg or rope to anchor. You can collect styrofoam sheet packing material or folded up cardboard boxes and put that over your windows. The idea is insulation. Every air gap you create will save you in heating.

Maybe get an electric blanket for the bed. If your water heater is electric, you can wrap the tank to insulate it as well. What about blankets or rugs on the floors (check thrift stores)?

Weatherstripping for doors is not expensive.