Moisture traps and dehumidifiers do one thing, draw moisture from the surrounding air.
In a sealed box that would be effective until the lid was opened.
In a caravan there are a lot of permanently open to the air vents which must never be blocked, and the air inside the van changes at a rate of about once per hour (very roughly) .
The whole purpose of the vents is to keep the van dry, and as long as you do not have any leaks they work perfectly well.
So, unless you are intent on attempting to dry out the whole world, they are useless, costly and , especially with moisture traps, if they overspill you have a problem with highly chemically charged liquid.