I have a 21ss and it has been my experience that if you are camping in either extreme, cold or hot, the furnace and ac are not very effective in the bunk ends. In the cold, the built in bed warmers are more effective for keeping you warm than the furnace. I even have 12 volt bed warmers (like truckers use in their cabs) for when we dry camp. The few times we have camped in high heat it was difficult to be comfortable in the bunk ends during the day, especially the rear bunk. At night it is better, especially with fans to pull the cool air into the bunk ends.
Michael, Cristina, Anthony, Cocoa Puff, Cookie, Blossom, Tosca, & Heidi
2012 Rockwood Roo 21SS