Also try putting some silicon grease on the bungs before you put them in. I'd recommend borrowing/ buying or hiring a dehumidifier. If you have a mains hookup, plug it in whilst parked into the the house and leave the dehumidifier on for a day or two. Keep emptying the container. Once you are confident it is dry, get someone to hose the area you think it's leaking around. Then use toilet paper to dab around, where it gets wet in an attempt to find out where the water is coming in from. *
Alternatively buy a can of stretch carpet spray glue. Then peel back the carpet which gets wet. You can then sit inside with a torch as someone hoses the rear of the van. It should be easier to trace water as you will see it running down the skin of the vehicle inside. The carpet can easily be replaced once you have sprayed glue on. If you have to temporarily remove any soft insulation, do so. You won't want it staying in there if it is wet anyway, as that could promote growth of mould.
The key point is to identify the leak's source so you can seal it properly. Keep in your mind, it could be leaking from more than one place.
* Plumbers use toilet paper to test for damp spots on pipework, as it is very absorbent and shows moisture very clearly. Don't worry about it disintegrating as once it is dry you can vacuum it off any carpet.