I use a couple of these FREEZE CUBEs in my two trailers. They have three or four different temp models. Mine is the one that activates at 35 Degrees and then turns off at 45 degrees. Available from AMAZON..
You can plug a oil filled portable heater into one or a couple of 100WATT drop lights here and there - open cabinet doors - storage bay etc...
Works good for us sitting in our camp back yard with power...
When camping in the mtns around here in the winter months we like to drain fresh water at the low points everywhere and use the BLOW OUT METHOD using our 12VDC tankless air compressor using an air adapter at the city water port and then just add some pink stuff to the P-traps. Will do this if the temps are going to get below freezing at nights. Then we can start using everything again by just adding water in the fresh water tank.
My off-road POPUP trailer only has the fresh water tank, sink, and hot water to deal with... Also the combo cassette potty and shower which is easy to dump all the water real easy when you need to...
Roy Ken