* Most likely location (for both light and heavy duty): Under the hood, usually on the driver's side fender apron or firewall. Look closely for a black plastic box with various fuses and possibly relays.
* Second location (possibly, depending on model and options): Inside the van, often under the dashboard on the driver's side. This would typically house smaller fuses for interior accessories.
To be absolutely certain, consult your 1985 Dodge Ram Van owner's manual. This is the best source for the exact location(s) and a fuse diagram. If you don't have the manual, you can often find a PDF version online through a search engine or a site specializing in vehicle manuals.