Dipping down to 30 for a couple hours shouldn't cause problems. How much do you trust the forecast to be accurate?
The water and stuff in the tanks probably wouldn't cause any problems even if it were to freeze; the tanks are somewhat flexible, and there's generally room for the water to expand upwards when it freezes. The water trapped in the plumbing between the tanks and the dump valve is far more likely to cause trouble if it freezes, possibly breaking that pipe. Under the conditions you describe, though, I doubt it would be at all likely to freeze hard at all.
Rock salt is one inexpensive way to help prevent freezing in the gray and black water tanks. It's probably not ideal for septic systems in large quantities, but at a developed campground dump station that would not concern me in the least (partly because it will get diluted a lot in short order). Obviously, salt water in the fresh water tank is not a great idea, even if it eliminates one step when cooking pasta.
