Well, it sounds like you have a bare wire, a loose connection or some type of short circuit that is causing this fuse to blow. An easy condition to correct once you find it. Its finding it that can be, in your mechanics words, costly. Very costly if you pay someone by the hour to do the work and they are not that sophisticated about DC (direct current) electricity. Since your mechanic gave up so quickly, I wouldn't even try to consult him on electrical issues again. Not his specialty, if you know what I mean.
If you are a do-it-yourselfer, here is what I suggest. Disassemble both sockets and disconnect the wires from both at the same time. Make sure any bare metal ends are insulated. Then reinstall a new fuse and see if it blows again. If it does, the problem is in the wiring somewhere deep in the bowels of the van. If not, the problem could be one of the sockets. Reconnect each one, one at a time and see if the fuse blows. If you find that connecting any one of the sockets causes the fuse to blow, you have found a problem socket that should be replaced.
Thinking about it, a short circuiting socket is about the only thing that would come on gradually like that. Did your mother have a habit of using either one of the sockets for an accessory (or lighting cigarettes) prior to the trouble. Most likely the one used the most is the problem.
Try this and let us know what you find or if you have questions.