You are only spossed to use the heaters BELOW 32 ambient. So the overall contents of the tanks will never reach 64 degrees, but the pad will turn off when the LOCAL area around the pad gets to 64 deg just as a precaution.
Honestly, 7 amp heaters are NOT very much heat in the overall scheme of things, but would prevent freezing of the tank contents down to about zero, depending on the insulation/sheathing around the tank.
The best way is to fully insulate the tanks and provide a furnace duct to the tank area, but I am guessing that the pads are less expensive.
There are similar pads of 110 volt varieties in use as block heaters, oil pan heaters, battery heaters etc.
I think they are a pretty good idea, but wouldn't even think about turning them on until the temp was around 20deg or so and I was hooked up to shore power.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME