I bought the Ultraheat dual voltage pads with the thought that if it is REALLY COLD (say -20*F)and I had 110V electricity available, I could tun both the 12V and the 110V side of the pads simultaneously, adding a little extra heat to the tanks. Also, when going down the road or if dry camping, I could use my on-board genny to power the pads in a similar manner.
I believe one of the previous posts said that a pad used about 8 or 9 amps DC and the battery load wasn't too bad. You'll probably have 3 of those pads (or more), so 24 to 27 amps is a considerable load if you are running on just batteries.