Looks like a bridge rectifier from an old-school charging system. But, I would have though more like that 1970s than the 1990s.
Other possibility is the speed-control resistors for the heating fan, but they usually live inside an air duct.
Formerly: 2005 Airstream Interstate (Sprinter 2500 T1N)
Now!: 2014 Great West Vans Legend SE (Sprinter 3500 NCV3 I4)