Turn the battery switches off and let it be. If you have the option, consider a trickle charger each for house and chassis batteries. Starting the engine and letting it idle may actually do more harm that good if you don't run the engine long enough/hard enough to bring it up to operating temp. If you plan to crank it while it's in storage, take it for a spin....say 30 miles or so to make sure that the engine/transmission warm up enough to boil off any condensation that builds up while sitting in storage. If not, then you're essentially circulating water throughout the engine.