Keith -
I'll defer to Ray on the freezing of the gray water tank. My comments were based on three nights in Pennsylvania with substantial snow and temps in the 20s, and two nights in New Hampshire, again with snow and 20's. However, in each case, there was no wind, which could make a big difference. No freezing in either case, and I still attribute at least some of this to dumping two gallons of warm dishwater into the tank in the evening.
You can buy glue-on heaters which slap onto the bottom of the tank, outside the trailer. They do require more than battery power, though, so they aren't much help if you are dry-camping. I've never had one, so can't tell you if they work, but the concept is good.
In the end, Ray's application of RV antifreeze is probably a safe precaution.
Bill
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