I've worked on the water lines quite a bit over the last 12 months (self inflicted damage). As far as I can tell, none of the lines run below the floor. The line from the water tank comes up into the cabin and stays there. If it gets too cold I'll be leaving a cabinet door open to ensure heat gets in there. But the way my trailer is laid out, a good portion of the water lines are in the cabinet right next to the heat ducts.

I've used space heaters when I can. I'm rarely in a campground with power hookups. So the space heater is only active when the generator is on. My solar panel is sufficiently effective that I don't really need to run the generator to charge the battery. The generator is used mainly to run the microwave. There have been a couple times where I ran the generator and the space heater to reduce my propane usage.

 

Here are a couple pictures of the Coroplast. I researched how to seal Coroplast and there are a lot of folks that have tried a number of different methods. The best result seems to be with a type of superglue from Loctite. I tried that but it didn't seem to be very reliable. So I ended up bending each corner so I could make a good solid fit. Each bent corner was screwed together and then sealed using Gorilla Glue Tape. I tested the tape at home on a piece of Coroplast and it stuck really well. Only time will tell how it handles weather.

This is a 120 gallon tank. It extends from behind the rear axle about 10" to in front of the front axle by about 45". Width is about 60".

Front of the tank. You can see the drain valve in the right front corner.


Rear of the tank

 

A couple of updates from my hunting camp which was the first couple of weeks in November. It got down to below zero one night and rarely got above freezing. It snowed at least 10", rnough that I piled snow around the windward end of the trailer to block the wind. In 10 days I went through more than two 30 lb propane tanks and 20 gallons of gasoline for the generator. There was limited sunshine so the solar panel wasn't producing much.

Reflectix does work. Sleeping next to a bare (single pane) window and then again next to the same window with Reflectix on it the difference is very noticable. If I can't have double pane windows, I'll use the Reflectix. For the price and the ease of installation, it's worth it. The Reflectix stays on my windows without any help. I tuck the edges into the window frame and it's not going anything. It survives the drive to/from camp without coming out.

I do have a skirt but when I had time to deploy it the wind was too high for one person to do it alone. Next year, the skirt goes on for sure. I'm going to put a small space heater under the fresh water tank just in case.

Given how cold it was, my insulation on the fresh water tank did well. But something still froze. I suspect it was the line between the tank and the pump. This was after about 7 days. It probably didn't help that the tank was low on water at that time. There was leak at the drain on the water heater that was not found for a few days. Whenever I ran the generator I tried to run the heating pads that are on the bottom of the fresh water tank. Either they aren't working at all or they simply aren't very effective.

Some things I wish I had done differently:
1. Replace the heating pads. Maybe they work and maybe they don't. But new ones should work. It's hard to test the pads since they don't come on unless the temperature is low enough.
2. Run heat tape along the hose that goes from the fresh water tank to the pump. Getting to the hose with the tank insulated will be very difficult. Uninstalling the Coroplast and insulation would be a lot of work.
3. Install something that allows me to keep track of the temperature of the water in the tank.

To keep the water in the tank from freezing I'm considering a way to cycle hot water back into the tank. Ideas so far:
1. Fill jugs with hot water from the kitchen sink and empty them back into the fresh water tank. Putting water into the fill port with anything other than a hose is very messy. This would be labor intensive.
2. Attach a hose to the low point drain pipe (Pex) and run it up into the fill port for the tank. I don't think this is reliable since the pipe hangs down a good 12" from the underbelly. The risk of the pipe freezing is high. Plus, opening the drain would lose some water. This is the easiest solution, assuming it actually works.
3. Cut the vent line from the fresh water tank, install a valve on the outgoing side (blue circle) and add a T fitting into the line. Finally I would tap into the hot water lines and run a spur off of that into the new T with a valve just before the T (red circle). The downside of this is that I'm not sure that the vent line can handle hot water. The vent and the fill lines are the same type of hose. I don't want to add this to the fill line because it would slow the water going into the tank - it's slow enough already.

 

What you describe is what some folks do with hot water side on shower line so as not to waste water going down the drain boondocking. Adding a T and line to fresh tank so the cold water (before hot water makes the distance) feeds back into the fresh tank. Depending on where your hot water tank is located to the fresh tank maybe there is a line near by?
Have you thought about using pipe insulation (comes in 3-4' lengths, split in the middle to go over pipe) on the water lines. Every little bit helps in those temps and if there was even a little breeze on a section it would freeze first and be hard to find until it sprang a leak.

 

The line from the fresh water tank to the pump isn't exposed. It's covered up with about the equivalent of R-9 insulation (rigid foam). I wouldn't think that adding pipe insulation would help that much, but if it's frozen the heat tape should do the job. Assuming I can actually get the heat tape on the line. The line from the fresh water tank is close to the tank's drain as I recall. I didn't put any protection on the drain and that was probably a mistake. If the tank is going to freeze, it's going to start at the drain. I'll be putting together a cap of sorts on the drain to give it a little protection from the cold.

The hot water tank is within maybe 5' of where the line comes up from the fresh water tank. There is a hot water line a bit closer than that. The vent line comes into the cabin through the same hole. So running hot water back into the fresh water tank shouldn't be all that difficult. I probably have most of the supplies already. You may be the scraprat, but I'm the packrat.

 

My "frozen" line from the water tank wasn't actually frozen. It turned out that the water pump was bad. New water pump installed and it fired right up.

During the 2020 hunting season it got down to -8° F and the fresh water tank did great. I froze a line on the other side of the trailer that ran past a poorly insulated section of wall. Judicious application of heat to that area got it unfrozen in an hour. During my recent wall repair, I eliminated an exterior shower, greatly improved the insulation, and added heat tape to that section just in case.