Dump your tanks before closing up. Then put about a half gallon or so of RV antifreeze in both the black and grey tanks. Need to try to get it down to the bottom of the drain near the gate valves for the dumps. Those are usually exposed and you really, really don't want them subject to expanding freezing water.
I have a small heating pad - for my shoulder at times - that I wrap around those valves and set on low. If I'm expecting temps down near 15 like north of Dallas is expecting - I'll wrap a AC duct sleeve around the area also.
Another concern is your propane tanks. Propane is less efficient at temps that low. I have one heating pad I use inside the cover on the tanks/ valves when temps are that low when we are in the rig.