So it looks like my leak is in the roof. Specifically in the back passenger corner near the back door. At first I thought that the leak was between the door seals. However, I found it strange that nothing near the top of the area was damp. The only area that appears damaged at all is the floor and the base of the cabinets.
The little strip of carpet between the cabinet and door was soaked and had some spores on it. There is water all across the back for the first half inch of the doorway. Then in that corner it is under an area that's about 18" square. And as you can see the base of the cabinet is pretty soaked also. The underflooring crumbles near the back and is pretty black. I don't see any damage to the van floor itself (so far). After pulling out the carpet I was a able to reach behind the paneling and feel the plastic shielding. It's damp back there. The paneling itself doesn't look damaged. So I'm thinking the leak is in the roof and it's running down the shielding/plastic sheet in the back and pooling on the carpet and rotting out the underflooring.
So now it's a matter of how I should fix it.
I noticed that the previous owner didn't use the rear latch to secure the roof down. Could this be the issue? (Really really hoping as it seems so easy). Or should I plan on getting some eternabond? Is this a matter of getting up there with some soapy water and looking for bubbles? I suspect the leak is where the roof sits on the van which is why I'm hoping it's about securing the back.
I will finish tearing out the carpet and underfloor when I get home tonight. I'm also thinking of just taking out that small cabinet. I'd lose two shelves and two drawers but it would open up that back area and allow me to haul some stuff around that would otherwise be awkward to pack. So carpet out, cabinet out, wipe it up, put in the heater to dry things out and spray the edges of the area with bleach to kill anything that may try to grow?
In happier news, I fixed two lights that had a lose wire and went and looked at my flooring options. Thinking that the allure laminate flooring that Home Depot sells may work well for this.