David, since you live in a dry environment, it must be pretty sunny during the day, yes? Are there any south-facing windows that will let in some sun during the day to help keep the place a little warmer? (assuming you are in the northern hemisphere)
Set jugs of water in the sun to collect heat, then when the sun goes, some of the heat is retained and released slowly. A true passive solar collector, in fact. The jugs of water, if left open at the top will also humidify your space somewhat as well.
I have a friend who heated a greenhouse in Salt Lake City (high, arid desert climate) for years mostly with just large storage tubs of water that warmed up in the daytime with the sun's heat, then kept the greenhouse warm at night. On the coldest nights he used a waterbed heater to keep the water warm.
I have Coleus in my garden that have survived a couple of winters. We do get a few cold nights here and there in winter, down into the 40's F and even an occasional venture near freezing. So, first of all humidity will help a lot, and secondly, put your Coleus plant up high when you're away during the day. On top of the fridge should stay pretty warm even when the rest of the place is cooling off. Kitchens and bathrooms are more humid than the rest of the house, too.
