I'm going to add a couple more thoughts on this topic.
Sam, if your trailer will be stored outside all winter, give it a good wax job..again if necessary. The better the wax job, the easier the trailer will be to clean in the Spring.
After you've taken care of the water lines, water heater and put anti-freeze down the traps and a bit into the gray/black water tanks, open all the cupboard doors, take the cushions into the house (or stand them up on the sides to get air around as much of the surface as possible), then put at least one container and crystals of Dry-A-Air in the trailer. Make certain you put the container on a piece of plastic (plastic bag works well) otherwise if the water overflows (because you forgot to check!) the "water" may pit and spoil whatever surface it's placed on. (Ask me how I found out about that
Also, because the idea is to draw the moisture out of the air, make certain the sink stopper is pushed in, or else you'll be drawing the moisture from the trap...do the same in the shower (if so equiped) and put the toilet seat and cover in the down position (okay you're a guy...maybe you wouldn't think to do this
If I do these things, my trailer is good to go for the winter months. I check the Dri-A-Air container every couple of weeks and dump out the water. Remember, if you don't do that often, all you're doing is recycling that moisture back into the air.
Here's an online link to Dri-A-Air
Depending on your location it should be available locally at nearly any big box store like Home Depot, Lowes, etc. I find I go through three bags of crystals in a winter. But, I also use two canisters, one in the sink and one on the floor up near the front.