Some RV's have enclosed holding tanks that are heated by the RV's furnace through heater ducting to the holding tank areas. As long as the furnace runs occasionally, the tanks won't freeze unless it's very cold (below 20F).
Read more: How to Cold Weather Camp in Your RV | eHow.com How to Cold Weather Camp in Your RV | eHow.com
Broadly, heated holding tanks are one of many steps to take if you want to use your coach in a cold weather area. I think I too have heated tanks, but haven't yet used them and have no plans to do so soon. We bought our 319TS in Dallas and drove it home 1500 miles to Southern California. We've been some local trips here in CA, but are planning to head for the East Coast one of these days, but along the I10 where we hope to avoid cold and, especially, snow.