forgot to winterize and it went to 21 last night

The best partial remedy is to open the low point and fresh tank drains, then open all faucets, including outdoor shower and indoor shower, kitchen and bathroom sinks. Hold toilet flush open for a minute or so. Then pour a cup or so of RV anti-freeze into all drains to protect traps. RV anti-freeze is available at all hardware stores, Walmarts, maybe auto supply stores. You may be okay just doing that as it doesn't stay that cold where you are.