Hi,
You have snow up there? Drizzle at Bach. this morning.
We also use our trailer mainly in the winter. And even with the enclosed and heated basement, I also have that problem......though not until temps drop into the low to mid teens for several days.
Since I usually dump at home into the septic tank, I just put a heater underneath for an hour or so prior to dumping. In cold weather (negative teens), I add an insulated box (open top) under the valve with a heater inside and let it cook.
I will have to give the hand warmer trick a try .... when the East coast folks stop hogging all the cold weather that is.
Steve