Hi to you, Greg,
Some here have suggested getting some bales of hay. Run a regular light
bulb (incandescent and not too big--60 Watts should work) under your trailer, and shove the bales of hay all around the perimeter.
You can wrap them in tarps first, or not...the hay acts as a thick insulation, the incandescent light
bulb puts out a surprising amount of heat.
We used to have an "outdoor dog" who was only 14 pounds and lived in a lightbulb-heated doghouse all winter and did fine--we did make a thick, insulated fabric door flap for him. Also, you could get some rolls of clear plastic and tape it over the windows
inside, though condensation might be an issue.
Our son lived in an "airstream" knock-off for a winter and did that--he went through a lot of heating propane
, though! And in WA state it didn't get down to 18 degrees that winter.
Boy, 18 degrees is hard to deal with--but check with a local feed store and see about the hay bales...
You could also get those silver insulating rolls and tape them around the bottom edge of the trailer...gotta do something to isolate the underside for keeping the warmth going. Do consider an industrial-type cord for the lightbulb--you could combine hay bales and that silver stuff...most big box hardware stores carry it...you can use that to wrap any exposed water lines outside the hay bale barrier...
Hope others chime in, many have had this issue!
Best luck with this! Frozen pipes are a mess in a trailer! (Well, OK, a mess anywhere!)