I think it the other way round, the heater causes the condensation, that and the cold surface of the van.
HiOne of the big problems causing your condensation in the van that needs attention is excess water.We have a couple of hozelok connectors sealed up with silicone adhesive that we put on the hose reels couplers before putting them into the van to prevent water dripping from the hose reel and leaving pools of water everywhere. We also drain the water out of the pole hose and give the poles a bit of a shake to get rid of any dripping water before we put the brushes into the van.We also drain the water out of the hose from the van top the hose reel when done and before we coil it up to put away.We store that sort of thing in a large plastic box to contain any water which gets washed out from time to time. And I also agree with ST1Allstar that you should leave your van windows open a tad to let the van breathe.Keeping the van as dry as possible reduces much of this condensation.
We have a couple of hozelok connectors sealed up with silicone adhesive that we put on the hose reels couplers before putting them into the van to prevent water dripping from the hose reel and leaving pools of water everywhere.