I ran my 55 1st panel heater motor on 12 volts for several years without problem. Having said that the safest way would be to hook it to six volts. I converted it to 12 volts and never thought about the heater motor. The gas gauge worked OK too. I'm not saying you should do the same, I may have just been lucky.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.