Fortunately, the tube thawed during the next day. That evening and all that night, I ran hot water into the tank every two hours, using my crude but effective recirculator:
Hot water recirculation technique
That method works, but it is not fun to get up out of a nice warm bed every two hours and stand there in the freezing dark for five minutes, while the hot water system warmed up the tank. The temp INSIDE the trailer was hovering in the low 30s. And this went on all week long. A fun, exciting, adventurous trip, but not exactly relaxing.
So, what to do? When we got home, a little googling uncovered this product:
Ultra Heat Self-Regulating Heat Cable
After discussing my project with the Ultra Heat tech staff, I decided to go ahead with this. (The tech I spoke with was Eric -- really helpful!!) . I was particularly worried about how much juice this would draw, since we usually boondock, and I don't want to kill the batteries. Eric told me that this thing is rated at 4 amps but usually draws much less (around 2 to 3). It also turns itself off when the ambient temperature is in the mid to high 30s.
They shipped the cable to our house -- quick delivery. As the product directions indicated, I cut it to length at a 45 degree angle and then put on a waterproof cap (included), which shrinks down when blasted with a heat gun.
Using cable ties, I then fastened the cable to the outflow tube. It was a lot harder than it sounds, because (of course) all of the work has to be done under the trailer, upside down. Worse yet, a good part of the tubing on my trailer is sandwiched between the top of the tank and the underside of the flooring in no man's land, so there is no way to see what you are doing. My hands were jammed up into that tiny gap.
So for that part of the project, I bought this little video camera at Home Depot:
Mini video camera
It worked! The lens snaked into the hidden area, and I could see the tubing and the cable ties on the video display. (I have since used the camera on other projects around the house.)
Here is what the tube and cable looked like, after using the cable ties but before installing the insulation:
On the left side of that shot, note that I used several cable ties at the point where the tube attaches to the water tank, in an effort to get the cable to conform to the contours of the elbow joint.
And this is what it looks like after I covered it all up with pipe insulation, per the instructions from Ultra Heat -- in this shot, my finger is on the part of tube where it connects to the tank:
This cable runs on 12 volt power. There are thin red and blue (positive and negative) wires running out of the end of the cable. The folks at Ultra Heat told me that this device is ordinarily run out of the converter. But when I checked, I could see that there were no extra fuse slots open on my converter. (It's a very small trailer, and I am sure that the manufacturer used the smallest possible converter.)
Also, I wanted the ability to control the flow of current to the cable, rather than relying on its ability to self-regulate. So, instead of running it to the converter, I powered it right from the battery. I ran ten gauge wire to the two leads on the heat cable, using waterproof wire nuts (and some extra duct tape -- why not?). I put ring terminals on the other end of the wires. Those ring terminals fit onto the posts of the battery.
The whole wire run is carefully secured under the trailer. In the evening, when I want to hook up the heat cable, I put the rings over the posts. In the morning, I remove the rings and tuck the wiring out of the way, inside the battery box.
Since I have two 110 amp/hour batteries, I think I have plenty of power to handle this load. (I try never to draw the batteries down below 12.1 volts. or 50 percent of capacity, or about 55 amp/hours.) . We recharge the batteries every day, either with our 120 watt portable solar panel or with the generator (seldom used). (Note that I am assuming that the temps during the day will rise above freezing. If not, the heat cable would have to run all day, and that would eventually deplete the battery, unless we have hookups.)
I thought that I would have to wait till our next cold weather high country camping trip to try this out, since the cable does not turn on until the temps drop into the low 30s. But surprise! We had a couple of nights in February where the temperature got down to 31 degrees in coastal Orange County, California, which almost never happens. I ran outside in the early morning, hooked up to the battery, and used the ammeter function on my multimeter to see if the cable was on. It was, and it was drawing just under two amps.
We've been hoping to go camping again to see if this device really does the job in real-world conditions. Unfortunately, there has been so much snow this season that none of our usual high-altitude boondocking spots can be reached yet. It will be May or June before the forest roads open up, and by then it will be too warm. My guess is that we will not really need this thing until next autumn.
I will update this post after we do some more cold weather camping!