You have a short circuit, try removing all of the lights then see if it still blows. If not put them back one by one to see where the fault lies. Look out for water ingress anywhere. The good news is dead faults are easy to find for an electrician, should be an easy fix.
Yes finding the fault will be relatively easy, but getting to it may be difficult.
sorting out problems with pre-existing wiring can sometimes be a nightmare
My own motorhome has some kind of problem with the cabling from the leisure battery +ve to the Sargent unit and it is not possible to access the cabling at all to investigate it more than a few inches before it disappears into a void. 
I'm quite attached to my dangly bits thanks Trev
Had to have a cooling unit replaced by Dometic South, we combined it with a trip to the area, even when it was in the workshop for the day we just took the bus into Tewkesbury. Our time lost due to repairs was an hour or so. If it requires a day or more for work to be done, that's a different matter.
...mmm. screws. you may be onto something there Merl. Put a couple through laminate flooring when putting a divider into a long under-seat storage area, which also houses some electric bits. Oh dearHas any work been carried out in the van at all? Very easy to put a screw in somewhere and hit a cable.
If you can borrow or buy a cheap multimeter with a tone function on the low ohms setting, basically a continuity tester with an audible indicator, with this you could connect the meter and leave it beeping while you jiggle stuff about, thumping walls and panels etc.
I'd remove the light fittings and check for trapped wires behind them before making a several hundred mile round trip. Good luck
reminds me of a conversion I ran some electrics for ..... had to run cables front to back along the floor (not a great idea but customers choice!) and I told the owner to make sure he made a note of where they were before he laid the floor down. week or so later got a call ... screws through cables! luckily I thought it might happen so I ran a spare cable at the time so told him to use that one instead
All I can say is that I'm happy for you Trev in your smugness.
Yes it can, when fitting alarm systems we regularly put a staple through a cable. The metal staple would act as a bridge between positive and negative. Only once in thirty years did removing the staple not solve the issue. I would try removing the screws and see how you get on. Possibly depending on the size of the screw things may be different here though, but you would be unfortunate if this did not solve your issue. But if you can access the cable it would be better if you could have a look at it and effect a repair if required.
Sorry Trev what do you mean bad places, they have to be run somewhere, and putting screws anywhere in a van or even your home carries risks. I am sure the cables have been ran appropriately by a well respected company like Devon, but unfortunately the poster may have accidentally damaged the cable by inserting a screw where a well run cable has been placed. Yes if you have wired your own van, and know exactly where all of your cable has been run that’s advantageous, but someone could buy your van and do as the poster has done, would you then say your cables were badly run.
