If the high level light is not working but the lower ones are then you are in luck as you know car electrics/coupling is good. If the bulbs are fine then I'd guess it is either the contacts in the fitting or I think the square grey? junction box that should be located either under the bed/dinette or under the van.
My money would be on the light fitting as if damp had got into the box then you expect to have other electrical gremlins.

Colin