We have a 2011 28RKWS 5er and have used it several times with the temps under 20 in the mountains of Colorado. If it gets below 20 we turn on our tank heaters and run our heat and have not had a problem. We do disconnect our water hose from the RV, or tend to leave a drip in our galley tank.
My DW is a camphost and we have hosted in several different Parks in Colorado to date.
We will be leaving Jan. 10, 2012 for Georgia (Fort Mountain SP) to host through the end of March.
We have had 4 different RV's and never had to use antifreeze as of yet! Knock on wood, lol!
Take care and have fun camping!