Condensation

A tub of rice ?

When you drop your phone in water, you throw the phone in some rice and it draws the moisture out of the phone.
my friend only just recently done this, and it works..
I told her about the rice.....and made sure I MEANT IT........ha ha

it should work inside the van...?

some moisture absorbent material would be commercially available..
at a price..

I'd check that water has not been getting in somewhere ... that much condensation could indicate there is water inside that is condensing when the sun warms up the camper

You can get some commercial moisture dehumidifier crystals in supermarkets for a couple of bucks. They will draw in the moisture and are pretty good. While living on the coast near a beach they prevented mould in the cupboards and wardrobes.

Disadvantage: Once the tub is full of water you need to chuck it so you will have to replace it every month or two (depends on how humid things are and the size of the tub of crystals). The tub will not overflow but once the crystals have dissolved the product is useless.

Advantage: They are not edible and attractive for mice.

I agree that you first need to check for leaks to ensure that is not the source of the problem

I think condensation is a common problem in campers. Ours came with a matt that sits under the mattress.
I can't remember what ours is called but there is one called Hypervent.

http://www.hyperventmarine.com/

Are you absolutely certain everything was bone dry to begin with?