
add air to the rear tyres, let out some air from the front tyes, that'll slope the van forwards and the condensation will roll forwards...away from the gear.
Seriously, condensation forms when there is moisture in the air, and moreso when the air is poorly circulated.
DIY stores sell various "passive" (Un-powered) products which absorb the moisture from the air. I've used the "tub traps" type with great results in the past, althought not in a vehicle. Tub traps, look like a large margarine tub, with a raised box-like air vent bit on the top.
However, you'll need to find a safe way of securing the mositure trap to your van - the moisture absorbing crystals slowly turn to a sludge which is caustic (burns skin, cant help gear much either) - the moisture traps instructions suggest that you only tip it down the toilet, rather than a sink. So, you'd need to get the cable ties, maybe a larger ice-cream tub container, sticky backed plastic and earn that Blue Peter badge.
The non-passive, (urmmm that'll be Active way then) is to improve air circulation in the van. Have a couple of 12volt fans covering the cargo area to do this.
Also rapid, wide ranging temperature changes will create, or add to condensation. If you currently pack away one lighting effect totally, then carry it to the van, then go back and pack away the next item totally, and take that to the van, those effects will be dropping from operating temperature to van temperature (bbbrrr) rapidly. If however, you pack everything away totally in the venue, ready for it to be taken out to the van, everything is cooling down to room temperature for a few minutes, before being dropped to van temperature. If you have the oppurtunity to have your roadie warm the van up for a few minutes too, before you open the final fire exit and take the gear out to the van, the gear wont be dropping much from room temperature much at all. This way the venue only has the fire exit open for a short time too, which they'll probably appreciate too.