Before you extend slides like the precious post stated you should put your jacks down, however if you are storing your motorhome you should leave your jacks up.
I would put a piece of plywood under all six tires that is to say pieces of plywood that fit the width of the tires. It could be 1/4" or 3/8" plywood.
Since it is a heated garage you shouldn't have to drain any water but I would so as not to have any bacteria form in tanks or lines and also in case of power outage.
Must be nice to have a heated garage for RV!
2014 Winnebago Adventurer 35P,Ford F-53, V-10
2011 Ford Escape,2000 Roadmaster Tow Dolly
"Have a Great Day, Enjoy RVing."