https://www.amazon.com/gp/aw/d/B071N...?ie=UTF8&psc=1
If you're going to run the suggested electric heater you'll find the gadget in the link above to be the thing Making the heater practical.
While almost all heaters have their own built in thermostat the above thermostat is remote, allowing the room to be at the temp you want at the place you want instead of the floor temp being 60 and the ceiling temp being 80.
We love this thing.
So easy.
And
My 24.1 has a wrap of heater duct around the water holding tank. It supplements.
And
Until a proof link is posted showing proof of tanks melting:
Ignore that tank melt thing.
And
Tank heater pads with the 40° built in thermostat are about 26 bucks and the easiest mod you'll do to your rv. Peel and stick and pigtail it to 12v power.