Joel, "survivability" is a subjective term and requires a level of determination to achieve. The word, in and of itself, implies some level of hardship in order to thrive and depends on an individual's threshold towards comfort. We have "survived" in a Bighorn 3670 through mild winters, say 20 deg. F nights, and warm summers with temperatures reaching highs of 110 degrees. In both cases we did not have a skirting system attached to the rig. We used a combination of electric heaters and the on-board gas furnace during the winter nights. The electric heat seemed to smooth out the cycle of heating and cooling typical of the gas furnace. It really helped us to us an electric blanket to minimize propane usage. I highly recommend two air conditioners. We are able to close off the bedroom and use the bedroom air conditioner to help make sleeping more comfortable in real warm summer evenings. We found it better to place the fan on the bedroom air conditioner off of auto and on to manual. This minimizes the on/off fan disturbance that might wake you up when air is cycled. For us, the slide out walls and floors are a big source of heat/cold loss. The entire floor is very cold in the winter and we used slippers to insulate our tootsies. We also used electric incandescent lamps in our basement to help keep pipes from freezing (be aware of fire dangers and examine your setup frequently). We used the on-board water tanks to prevent water feeds from freezing. We also filled grey and black water tanks and dumped, when full, to avoid sewer lines freezing. So, yes, you can survive in various weather but it will take experience to deal with insulating pipes. I have absolutely no idea what various manufactures use to determine whether or not a rig is "4-season". Some will say dual pane windows, "more" insulation, heated tanks, etc., but I think the biggest advantage towards 4-season rig-living will be your ability to adapt to the reality of the circumstances. Best of luck.