There is so much misinformation here, I don't know where to start...
Propane furnaces (as opposed to propane catalytic heaters or propane stoves) do not add any humidity, nor does a heat pump. The are closed systems and they add no water to the camper. People add humidity by breathing primarily and that is why you need to control humidity either by opening a window or by a dehumidifier. Since Campers are pretty small compared to houses, the humidity builds up faster. Lower R values also contribute to the problem as the cold walls and windows condense moisture at lower humidity levels and attract mold growth.
In humid ares during the winter, it is preferrable do take all measure possible to reduce the relative humidity to 50% or a bit lower and that can be pretty difficult.
2007 Chevy 2500HD Classic LBZ Duramax/Allison (Stock)
2005 Bigfoot 1500C8.2 AWESOME