[QUOTE=DeltaRon;1701899]1. Should I leave a vent open slightly all winter (fantastic fan, i.e.)?
2. How do you protect black/grey dump valves? Was thinking about dumping a cup or so of anti-freeze down drain and toilet."
A Fantastic-Vent Fan is only guaranteed against wind damage when the vent lid is all the way open or all the way closed. Partially opened, a wind gust can break the vent's opening/closing mechanism. The newer automatic versions only go full open or completely shut.
Winterizing the gray and black tanks is best done by draining those tanks. If you will feel more secure, or think that maybe there is some small amount of water left in the tanks, then pour a gallon or so of anti-freeze down the sink drain and also a gallon into the toilet & flush it. You don't need to use the pink RV antifreeze @ $5 per gallon in the gray and black tanks. You can use blue automotive windshield fluid, but be sure it is rated for below zero use. There are some summertime versions that freeze at 32 degrees. Don't use that kind. The anti-freeze will automatically sink to the lowest points in the tanks and protect the drain valves. The blue antifreeze usually only costs $1 to $1.50 per gallon. Don't ever forget and use it in the fresh water lines or fresh water tank. If you think you could ever confuse the two, then just stick with the pink RV anti-freeze.
A trailer that is winterized and not heated probably won't generate much interior humidity. Usually, only occupied trailers develop humidity problems in the wintertime. My opinion: having an open window or vent is not as important as keeping out pests during the storage.
I don't keep any windows open on my trailers during the winter. I winterize them, keep a dehumidifier running, use an electronic anti-pest squawker, and set an electric ceramic heater in each trailer on the anti-freeze setting.
I plan to be in Destin and Pensacola from Nov. to May this year, so I know one trailer won't be winterized this winter.
Let's Roll !