That's a tough call. I don't that is block cracking cold, but it's cold enough for cause of concern.
I guess if it was me, I would get all the water I can out of all the tanks. Then put a little bit of RV antifreeze in the tank. Flush it in the spring and your good to go...
As far as...
Everyone's interputation of "freezing" is different...are we talking 30* cold or -10* cold?
I have been known to throw a bottle of Iso/alachol in my old motorhome's water tank on hunting trips to keep it from freezing on the way home. It works until I can dump out the standing water...