I find the best cases for truck storage are the traditional fleece lined ones. These seem to keep the rifle at a good ambient temp and wick off any condensation. I use a long fleece-lined "sock" stlye case from Dixie Gun Works for my long Mosins and a smaller side opening fleece-lined bag for my shorter rifles. For my Mossberg I just use an assault rifle case but then again a bit of rust on that beasty isn't a problem.
I've tried the "no case" option but unless I bring it in and rub it down every other day or so Alaska will have her way with the rifle. The inside of the truck cab temp changes from as cold as twenty below zero to fifty above as it freezes up and thaws out every day, which causes a truly amazing amount of condensation and seems to rust the bejesus out of anything in the cab. The only way to prevent this would be to lock the rifle in an unheated box in the bed, which I know some folks do. This solves the problem of massive temp. variations.