I have two swtiches for the 12 & 24 volt. Both were on when I tried starting the bus. I do keep them in the off position when not in use. So I know for sure that is not going to be an issue.
Unless you need to move the bus, I would just leave it alone until the weather warmed up again. There is no need to run a Diesel every week, every other week or even every month. Things just don't happen in a Diesel like it does in a gasoline engine. There is nothing harder on an engine then starting or cranking it in sub freezing weather where the oil is very thick. Once again, unless you have to move it-leave the engine alone until the weather is back up in the 70's. Good Luck, TomC