Same problem with skirts on my '08 845 ... the interior of the wing was weak and some water intrusion and rot.
I bought a piece of marine 3/4 plywood, cut a new panel to fit each side, drilled it out for lights and water drains. Sealer and paint on both sides and all edges,and installed it (in process now) and will share photos here when finished. Just got some exterior white paint, and good as new with little chance for rot now.
I did notice the caulking was "a little weak" in application all around that area. For the future, I'll post photos later too. I discovered the front jack mounts had some 'give' in them when loading ... so, I welded a bracket that lays alongside the camper wall, and drilled bolted it thru - takes most of the stress off those very tall jacks in front. Now, it's solid, and not the give it had before --
Monkey44
Cape Cod Ma & Central Fla
Chevy 2500HD 4x4 DC-SB
2008 Lance 845
Back-country camping fanatic