Mike, this is the voice of experience talking. We bought a tank heater on line, $99, no tax, freight included. I don't know anywhere near as much about RV'ing as a few of our members, but this problem has reared its ugly head one too many times with our rig. The blanket heater solved all the problems.
Here is a similar heater:
https://www.northerntool.com/shop/to...iABEgJlT_D_BwE