Very sensible Allan, however, all the tests I suggested with the multimeter were without mains connected to the van. Also, rewiring the switch, to check the fuse holder is not at fault, can be done while disconnected from mains and then reconnected to test when your hands are well out of the way.
I'll also add, the switches wiring needs checking, last one I did with a very similar problem, turned out to be a replacement switch had the neutral wire in the switch badly connected, ie, the screw was clamping the insulation!
PS, Birdman, no worries and your not alone, even my bank excepts cheques made out to ARK?!