It has been pointed out that this can occur in VW`s and CI`s and other coachbuilts. Foam mattresses seem to be the culprits but I go back to my original statement that the overall ventilation in the van may be an underlying factor.
When we breathe, we give out water vapour, Cooking with gas gives off the same and boiling kettles are bad for moisture. Add in that cold damp conditions outside the van will soon affect a badly insulated vehicle and you are bound to get problems.
As this vehicle is a converted bus, is it single glazed (at least in part)?
Do the windows suffer greatly with condensation?
Is there any dampness or musty smells in cupboards etc.?
One thing you could try is a small 12v fan. Either fit one behind the gas heater (computer fan) to circulate heat or to blow across the front of the heater to do the same job.
If you are not using the van much in the Winter, it is general advice for all vans to remove soft furnishings and keep them in the house or somewhere dry. In your case, I would do this and use a dehumidifier regularly to keep dampness at bay.