Originally Posted by WeBJammin'
The Home Depot sells a 100 foot 12/3 conductor heavy duty extension cord that would easily make the run you are talking about.
Forget the propane heater inside and either run an electric powered heater (resistance in the cord will be lower if temps are low outside) or use air to blow out the lines on the trailer.
Personally I would just use air to blow out the lines, drain the water heater and add a little antifreeze to the P-traps. Doesn't take long, nothing to flush out when reusing and no chance of a fire.