Don't laugh but we have solved a few problems for people using rope light. Normal heat tape uses 18 watts per foot which is way overkill for an insulated tank compartment. Rope light (not the LED type) is 3 watts per foot. You don't need much battery and solar capacity to supply that kind of load. Take a 10 foot piece and locate in the insulated tank enclosure and along any more exposed spots. If properly enclosed you would be surprised how warm it gets in there. Use a remote wireless thermometer to keep track of things. It may not work for you but we have solved a few problems for friends using this approach.
hope this helps.
John
