Welcome to the forum and your plan is doable, but it is going to take some work and a bunch of $$$$$$$. If your plan is to do this for work, in the general area where you are going to camp, great. If your plan is to do this and cut your costs down and save some $$$, that is a mistake. I have lived in park service trailers that were prepped for subzero weather and we still needed to take steps for long durations of cold. All parts of Utah will have those type of nights. Like many parts of the west, things will warm up in the daytime, but once the long nights come, it will be cold and for the size of a Toy Hauler, well, I see a number of problems. 1. Two small of propane for that much of square footage and high ceilings. 2. Type 27 batteries will get hammered by the furnace and other uses. 3. Water usesage and “Bail Plan” for those unexpected nights of drastic temperature drops. What are your plans, do you have a compressor? Do you have enough knowledge and familiarity of your TH to know before it is too late to pull the plug?
What puzzles me is you shared you have 20 years of experience in doing this and just the school of hard knocks should provide you a good start already. You mentioned you have put insulation on the pipes. That is good to a point, if you have warm water or heat running through them. Think of how a thermos bottle works. If you have something cold inside it, the thermos keeps it cold, Warm it keeps it warm. So the insulation can actually work against you.
I don’t recommend you follow the advice shared above in turning down the temps for cold weather. This is good advice if you have your water system blown out. If you have your system still charged, this is wrong advice. At night you should actually turn the heat up. This keeps the cold creep out of your trailer. Think of an onion. As the cold gets colder outside, the rings where the heater is can only struggle to stay warm and the outer rings get invaded by the cold, layer by layer.
Winter time it is important to be aware of humidity. If you stop breathing, you are not putting moisture in the air, as other things are too. :B Pets, propane heaters, showers, cooking. When you can, shower early in the day, on a sunny day. Get the towels and anything wet, outside to dry. Same for cold and wet clothes. Get them out of the TH. Cook early in the day, as it also heats the TH up in the morning too. Move all clothes, bedding and organic matter out of cabinets and away from the walls. Stuff bags makes it easy to do this. Weekenders are not going to see this, as they allow the camper to dry out between trips. If you plan on staying in yours, you will see this. Whenever you can, open up the windows and vents on Sunny days. Get some air coming and going.
Yes, solar is great! Love it. But, you need a plan and you already said your budget is small. Bummer. Spend the money for some better batteries and an auxiliary battery charger. Learn more about charging batteries effectively and follow a regime. It can be done, I did it.
Good luck and there is a lot of really good information and people here. Just be sure to weigh the information of whether it is from someone with experience in what you are doing, or planning to do.
Again, welcome to the forum.
b