... since I believe you currently run the engine each day for the reefer, I understand it's simple & inexpensive to drop in a car heater which would take the chill of in the AM which is when you're presumably running the engine. Hamilton carries them:
http://store.hamiltonmarine.com/browse.cfm/heater-28-000-btu-12v-flush-mnt-w-switch-wire-117075/4,220.html

I like the Mr Heater suggestion, may check it out myself.

In the past on this board people have raved about the Wallis heater; if I ever cruise in a northern climate I will likely put one aboard. I realize it's beyond the scope of what you're looking for here.

http://www.scanmarineusa.com/wallas_furnace_30d.pdf

Cheers John,
Max