Mold on dinette frame under cushions

What do the tips of the screws look like? Less rust, more rust, or about the same as the area near the head? If there is less rust towards the tip, that may indicate that the wall itself is dry and the source of the moisture is somewhere in the camper interior.

Of course that only works if the screws are threaded into wood. But I believe there is still a lot of structural wood in the newer Adventurer campers.

When I look at your photos, it helps to narrow down the possible issues. Based on what I see, it looks like picture number one reveals what was under the seat cushions, and picture number two shows mold on both sides of your hot water tank enclosure. BUT it also seems like there's mold higher up.

If that mold is indeed higher and your hot water tank was sealed in with the plywood AND cushions, I suspect water came from above. Is there any way to use a camera or cell phone to see what's behind that wall board?

If the mold was just under the seat, it could suggest a leak in one of the lines to the hot water tank, spraying water around. But if there's no mold on the floor under the seat or signs of water, like disturbed dust with a trail, and the mold is higher up, it likely came from above. This would prompt me to inspect the roof for any damage or missing parts, like a cap for a fridge vent.

is the area vented?  warm humid and no ventalation is a recipie for mold.  if you can find a way to add a couple vents to the area so there will be some air movment that should reduce the issue.  

 
(Shot while screw removal in progress)

photos to help illustrate:

I believe that having so much mold after only being used six times isn't likely due to camper usage. It seems more probable that water seeped in from outside.

Water usually doesn't cause mold unless it stays for a long time or gets repeatedly wet. Even if it was condensation from the heater, which I've never seen, it doesn't add up, especially considering the limited usage by the original owner.

I suspect water might have entered through a window, running behind the walls or elsewhere without leaving visible marks. Water needs to come into contact with wood multiple times or soak in to leave a visible mark.

Again based on my experience, I find it hard to believe that such mold would result from only six uses of the vehicle.

Second thoughts -
COULD it be from the original owner leaving something wet in there (folding kayak, skis, etc)  OR leaving a window open and water getting in that way, or is a cap missing on the roof for the fridge???