Going a bit tangential; but what about the old, camper shell ammonia reefers?
I seemta recall that they took no power..were all gravity/ambient temp operated??
A co-worker had an old truck camper with what appeared to be an ice chest. Some old fella tuned him into what-was-what and all he needed was to turn it upside-down for a few days and it worked again!
Am I wrong in that rememberance? Is there suchan animal? I *know* there's propane-heated reefers....lotsa Amish and RV dealers about here
If ther'd be a no power/heat unit; that would be tthe cat's cojones