It’s a drag when you go to empty your black or grey tanks only to find the valve is frozen. A quick fix is run a hair dryer on the valve for a few minutes. Most RVer’s will have an extension cord, and you could always fire up the generator if there’s not a handy 110v socket near you.
If this doesn’t work, then your tanks or pipes leading out of them may be frozen. For that, you’ll likely need tank heaters.
Another trick I recently learned about was to put salt pellets down the black and grey water tanks. This will help keep the tanks from freezing and should have no adverse side effects that I can think of, except of course your poop water may taste salty. Eeeewww!