Once evacuated, flushed and the valve closed, add a gallon or two of RV anti-freeze or even windshield washer fluid to the tank through the toilet. Be sure the strength is applicable for the expected temperatures though. And be sure it is RV anti-freeze and that the windshield fluid is permitted to be dumped into your septic system. If possible, see if you can add insulation around all sides of the holding tank. In sub-freezing temps, it is difficult, but doable is most cases to continue to use the on-board waste systems.
Unfortunately, the only other alternative I can think of contain the words “wait,” “spring” and “thaw” in it. If you have enclosed holding tanks, you can place a 100-watt trouble light inside the compartment to help keep it warm after you finally get it evacuated completely. Also consider purchasing an aftermarket holding tank heating pad prior to, say, next September.