We live in a converted school bus in a campground. We use a waterbed heater
on each of our holding tanks (30 gallon fresh & 25 gallon black... 70 gallon grey stays open when on full hookups) so our tanks do not freeze. Tanks sit in insulated boxes. The waterbed heater is cheap, works great and I have it on an EasyHeat EH-38
plug in thermostat (I bought it several years back at either Lowes or ACE Hardware) so the whole thing doesn't get any power until temps are getting close to freezing. We set the heater pad as low as we can as we don't want warm water. I will need to add another waterbed pad when we add the extra 30 gallon fresh tank (will give us 70 gallons of fresh water since we filter water into our fresh and draw water 24/7 out of it).
There is an RV heater pad
that heats a 15 gallon tank. It is 12vDC/120vAC.
The heat pipe cable will not do what you want it to do. The heat pads will. This
is what we use heat pipe cable for!