Mold on dinette frame under cushions

[PNWtruckCamper]

We recently purchased a new-to-us 2020 Adventurer 89RB, used all of 6 times by the original owner. While looking in all the nooks and crannies to get to know our new truck camper, we discovered surface mold on the wooden seat base under the cushion of the left-hand dinette, along the outside wall and a little into the corner by the fridge. Unscrewing the panel uncovered mold along the top and bottom of the wood frame structure that the wooden seat base sits on, in the same areas (along the outside wall and onto the fridge wall). The cavity underneath that dinette seat holds the hot water heater and a duct from the fridge. Close inspection shows no watermarks on the window or wall above that area, nor signs of drips or pooling underneath or around the water heater. 

Could it be that the water heater itself is a source of condensation in that area? 

Has anyone else had mold in that area and know what caused it? 

The goal is to address whatever the source of it is and prevent it from recurring in the future...

[Grit_dog]

There could be a localized source of additional moisture/humidity, but just as likely simply my previous reply. 
kill the mold and leave that seat/panel open when in storage. (and keep some active ventilation going in it during the wet season)

If you’re storing it outside in the wet season,  a breathable cover will do wonders. 
May not eliminate the need 100% to keep the air moving in it, but stored a RV outdoors here and in SE AK too for a few years and the cover kept all condensation and moisture away. Never had to run fans if they had breathable covers on them. 

[Photomike]

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???