Here is a picture of the label of a tank heater pad. It shows the voltage 13.5 vdc, amps 4.8A and watts 65W.
In my opinion, the built in thermostat should be set to come on at 36⁰F and off at 40⁰F.
That would save the batteries from unnecessarily running the heater pads.
I bought a digital temperature controller for operating the RV fridge exterior cooling fans
I guess you could wire one in to control the heater pads to come on/off when you want them to.
HiLetgo DC 12V 10A Digital LED Temperature Controller XH-W3001 Mini Thermostat -50 to 110 Degree Heating/Cooling Temperature Control Switch with Waterproof Sensor Probe
https://a.co/d/aVKR5Vd
The real problem with being off grid is power.
I'm in a FB group Solar Powered RV's and Boondocking and one guy posted he had a 2,200W system that could run everything. It cost him $20,000.
Personally generators are cheaper but you have to haul gas.