Never used FW tank, need to sanitize for use next week, how?

Thanks again for the help, guys! I filled the tank with a chlorine solution, ran the pump to get it into the water lines inside the rig, let it sit for a few hours, drained it, flushed with clean water, drained again, flushed with a baking soda solution, and drained it. I will wait to refill it at the campground. The temperature is supposed to get down to 4 degrees on Saturday night, and a high of only 21 degrees on Sunday, so I'm afraid we may really need the FW tank.