Consider a Kozy-World propane brick heater. We bought a one-brick heater to use while boondocking in Quartzsite and have used it many times since. In fact, it's on right now because our transfer switch needs to be replaced (on order). We bring the propane hose through the window (and use towels in the open space) and it keeps the rv warm, even when the temps are in the 30's. Of course, if we had to use it all the time, we'd come up with a better solution to the partially open window. Our is a basic model, low and high only, but there are other models (two- and three- brick) that have thermostats on them. Bought ours in 2004 at a camping store in Quartzsite, can't remember the name. Look on-line.