I use the blow out method here which includes draining the water tank, hot water heater tank, and toilet tank. Then I use a 12VDC tankless compressor to blow out all of the water in the city water lines using an air adapter feeding the city water port. Then I pour some pink stuff down the p-traps in the sink.
Don't forget any of the fresh water cans you have that still may have some fresh water in them...
Been doing since owning my POPUP Trailer since buying new in 2008.
It sits outside in its designated spot when not is use.
Have not had any issues except I usually need to get rid of a few mud dobbers nests in the spring. They love to build nest inside the propane cover areas and under the trailer frames... I also have all the holes that allows insects passage from the bottom side of the trailer sealed with RTV...
I have one of those THERMO CUBES that plugs into a standard 120VAC receptacle inside my popup trailer where i have a oil-filled low profile heater plugged into it. This comes on at 35 degrees and turns off at 45 degrees.. You could even get away with just a 100WATT incandescent light bulb instead of a oil-filled heater since this area is closed off...
My two RV trailers are both plugged in 120VAC 20AMP garage receptacle sitting in the yard when not in use keeping my battery banks trickle charged during the winter months.
Roy Ken