My advice... Buy a ~$40 battery load tester from your local auto parts store before you go buying batteries. The tester will tell you if the batteries are totally gone or salvageable and just need a charge.
I work on vehicles like this on a daily basis. It is completely reasonable for the batteries to go bad just sitting. If any plow Hookups or auxiliary lights/solenoids are wired to the battery it is possible for them to draw power even when not in use.
Posted via Mobile Device