Amy, I'm sorry you're having trouble in the cold weather.
Before answering your question, I need to ask ... do you live in cold country ... that is, do you have freezing temps at home? If so, (assuming you were using your water system in the freezing weather) and you don't store your rig in a heated facility, you'll have to winterize it to keep pipes & such from bursting. You can have that done at a shop if you don't know how to do it.In answer to your question, I have another question: Are your black and grey tank valves on the exterior or in their own bay? Mine are in a bay, and I run an extension cord to a clamp-on light in the bay to keep it from freezing. The heat from the light bulb is just enough to keep the compartment above freezing (which is all you need). I don't know which throw off more heat ... the regular filament or halogen bulbs.
With the external valves, I don't know how to keep them from freezing.
To deal with your frozen valve, I have a
"Flush King", which uses pressure from a water hose to create back-pressure to rinse out the black tank & keep sludge from building up. You hook the Flush King on where you normally hook your sewer hose, and the sewer hose hooks onto the bottom of the Flush King. Perhaps you could hook the Flush King up to a warm/hot water valve to melt or knock loose the part that is frozen (I doubt the whole tank is frozen).
Here are some more holding-tank tips: Try to get your grey tank as full as possible before dumping your black tank. Empty the black tank first, then grey. The grey water rinses the black tank "gunk" (for lack of a better word) out of the hose. In freezing weather be sure **not** to keep your sewer hose attached (gosh, you can be SO sorry!
). Only hook it up to dump your tanks, then disconnect it and allow it to drain before putting it away.
I apologize if you already know these things ... Good luck & let us know what you find out.
(EDITED SEVERAL TIMES FOR MORE CLARITY ... )