Would be nice to have a box of these and extra duct tape on hand in the event they are needed:
6 Patches Of Hand Warmer Heating Pad-4 Minutes Heat up for 14 HoursJust not sure if there is easy access to the exterior pipe from your pump that goes into the fresh tank.
I also wonder if one of these pads taped to your outside covered jerry can would be enough to keep it from freezing. At least the pads do not require a powered source for heat.
There is also this very small DC powered heating "pad" (10cm X 5cm) that can draw less than 1 Amp. That might be low enough to run on your existing power. Towards the bottom of the item page there is a link for spec sheet where the power information is available.
Electric Heating Pad - 10cm x 5cm
