Ray, if you feel the thermostat has failed, HD and LOWES have them for about $30 in the roofing supplies section.
If you wish to use a humidistat, you will have to order one online or through a local supply house. Units with both temp and humidity controls are kinda spendy and available through most greenhouse supply vendors and that would be my choice without question.
Frost should not be a problem in the attic as long as it is just frost. If you have ICE buildup in the attic space, then you have other problems. (Poor attic venitllation and/or poor attic insulation).
Much of the moisture content in the home escapes through the attic. Your fish tanks and showers are a major contributor to water vapor in the house. Most of us with fish tanks don't have overly dry homes in the winter and can actually have moisture problems even during the dryest months of the year. Always use the bathroom fans and range hood to remove water vapor from showers and when cooking. It can make a huge difference.
With regard to the fan in the winter... as Paul pointed out, it will not hurt. In fact, if your attic is not well ventillated and/or your ceiling not well insulated, the fan will help. Placing it on a timer is certainly an option.
If the snow on your roof melts before all of your neighbors, you have a possible problem, as the heat from the house and/or the sun is heating the attic space and melting the snow from the bottom up. This is what causes ice dams and leads to roof damage. As Paul mentioned, the ideal would be to have the attic space at the same temperature as the outside air
This will help prevent condenstation and ice dams.
From the sound of it, your home does not have a ridge vent or eave vents and instead has a fan and a gable vent. Before you get worried and decide to change anything (other than replacing the thermostat with a combo thermostat/humidistat), keep an eye on the attic space and roof as the witner starts.
