We camp all the time in our off-road POPUP camper which has a fresh water tank and inside plumbing for that as well as a hot water water heater etc... We also do not have any under belly membrane covering.
We do just fine by unhooking any water connections and running heat inside the trailer on the very cold nights.
We also are setup for doing a quick drain of the trailer fresh water low points which also drains the hot water water tank to some degree if we open the hot water faucet. Once all of the water gets drained from the low water point we can do a quick blow-out of the water lines using our tankless 12VDC air compressor connected to the city water inlet port using an air adapter... Then we just add some pink stuff to the couple of p-traps we have.
All of this just takes a few minutes and we can start using the water system again by just adding fresh water the next day to the fresh water tank or hook back up to city water.
Have camped thru some low single digit temps doing this here in the high country of VA-TN-NC-GA camp sites.
We also have all of our cabinets that are interconnected around our floorspace using internal small vents and use a 12VDC vent fan on one end to force air movement through all of the cabinets. this helps big time keeping our internal warm air inside our cabinets...
Not really a big deal for us camping during the winter months... Having a bigger trailer might be more of a problem...
Our winters here in Northern Neck Virginia are not harsh at all - It may get down to freezing overnite but usually warms back up the next day.
The only thing that has ever frozen up for us water forgetting to unhook the water hose connection to the camp ground pedestal one cold night.
Roy Ken