I wouldn't sweat the upper 20's too much. If it hits 50 during the day it's only going to be below freezing for a few hours a night. It takes more than that to hard freeze plumbing. I camp a lot when it's that temp and just use a space heater to heat with, and disconnect the city water at night. You could always let a faucet slow drip at night, but you will fill up your gray water tank fairly quickly. If you have one of those portable sewer totes, that may not be a bad option.
If you flood your camper, don't blame me.