What do you do for condensation inside cabinets?

Does the van run?
Drive somewhere it's not raining?
Dunno what your van looks like but diy conversion could mean about anything.
It's hard to keep moisture down in a full size truck camper, with good insulation, forced air heat, if there's multiple people and it's wet/winter weather, wet clothes etc.
Keep air circulating with a fan and get a dehumidifier if that doesn't work.