You could try purchasing 12V heating pads used to heat the black and gray tanks and wrap them around the water lines. Then you could heat the lines even when on battery power by flipping a switch.
They come in different sizes. Here is one example:
https://www.amazon.com/Heater-Holdin...07ZJN2JW7&th=1
If you wanted to go with AC instead of DC, you could try radiant floor heating mat but you would need to rig up a thermostat for it:
https://www.amazon.com/dp/B083XVP9CN...jaz10cnVl&th=1
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