I have been in my Designer full time since August 2010. I do have the Polar package and dual pane windows. The highest temp I have seen so far was in August and in the mid 90's. Both a/c's work fine. No problems maintaining a comfortable temp inside.
The lowest temp I have seen was 13 degrees a few weeks ago and the furnace had no problems keeping it very comfortable in the living area. I also have the electric "fireplace" and I do use it also to keep the propane usage down a bit, but comfortable it definitely is even without the electric heater.
2006 GMC 2500HD CCSB 4x4 Duramax/Allison, Titan 52 gallon fuel tank, Prodigy Controller, B&W Companion Hitch
2010 Jayco Designer 35RLTS, Cummins/Onan RV QG 5500 EVAP, Progressive Industries EMS-PT50C, TST Systems 507 TPMS, RV Flex Armor Roof