Whether 12v or 120v, they would be a very low amp draw. I had an engine heater on my airplane (in the day) and it was way more substantial than the tank heaters yet only drew about 6 amps, so you are fine on 15-20 amp house circuit as long as you use an appropriately heavy extension cord. I keep my plugged to a 20 amp circuit all winter (I do winterize, but for keeping house batteries up) and it will even run the roof heat pump which I believe pulls around 13-14 amps during start up. I should check that I guess.
2016 Jayco GreyHawk 31DS
2016 Jeep Cherokee Trailhawk Toad.
SE Washington State