This isn't 6.0 specific, but for any vehicle I recommend a digital voltmeter (they're really cheap now) or test light; hookup wire (18 or 14 ga "secondary" and 12 or 10 ga "primary" wire from an auto parts store is good); crimp connectors (butt, male and female spade, and ring) for both wire sizes; wire crimpers, strippers, and cutters (there are tools that will do all three.); and black electrical tape.
These tools don't take up much space and will allow you to troubleshoot and at least temporarily repair a lot of electrical problems, both in the chassis electrics and in the camping equipment. I've made a lot more electrical repairs on the road than I have mechanical repairs.
Oh, and spare fuses. You may already have a couple in the fuse box, but if your van has been around a while someone may have already used them.
Also, I firmly believe that every vehicle, whether it's a camper or a passenger car, should have a good flashlight.
1990 E-250 Sportsmobile w/ penthouse top, converted when new by SMB Texas.