P30 Step Van HELP!!

Wasn't sure if I should post this in the electrical section cause unfortunately it is not a MOPAR that I am working on, I had to take what I could get though.

I'm having electrical problems with my rig. It keeps blowing the parking light fuse so I end up with no tail lights, no markers and no parking lights. I don't feel very safe driving it without them.

So this week I took out all the light bulbs in the markers and parking lights, I turn on just the parking lights and started adding bulbs back one at a time to see if there was one fixture that was causing the problem. I did end up blowing the fuse on the top center marking light so I left those bulbs out and got everything to come back on. On my way back the the storage facility I hit a pretty big bump in the road and all the lights went out again, the fuse blew. I decided that all the marker lights were just old and that I should just go ahead and replace all the fixtures instead of wasting all the time to try and find another fixture that was bad. So I buy the $30 in lights and replace them, I replace the fuse turn on the lights expecting to see a nicely lit rig and the fuse blows out, I go back and remove all the bulbs again and replace the fuse and it blows again?? So now I have all new lights and I can't get a fuse to stay good even without bulbs?? Anyone out there have any good advice on what I should check now? All help greatly appreciated as I am getting a little frustrated.