If the batteries are going dead in 3 or 4 days they weren't fully charged to begin with. There is a little draw on the battery but it should start after weeks of sitting and not being driven.
As far as undercoating. With the corrosion protection these vehicles get at the factory, aftermarket undercoating is unnecessary.
As far as undercoating. With the corrosion protection these vehicles get at the factory, aftermarket undercoating is unnecessary.