An extremely common place on a VW Camper very prone to rusting is is the Battery Seat. This can be caused by the battery leaking acid aswell as being a bit of a water and leaf trap underneath.
This is located in the engine bay on the right hand side. To check this is a Must. Its a bit of a pain as the battery has to be removed. But if you are spending ££££ on a VW Camper i woiuld seriously recommend spending at least 5 minutes removing it. If the seller is selling a genuine van they will have no problems removing this to show you how good their van is.
When check the tray under the battery check the exact same area on the left side. Although this is a lot less prone to rusting.
VW Eurovan Camper Battery box,