Corner repair jayco kiwi

[Al0311]

disclaimer I’m pretty new to the rv camper world But I have repaired boat floors and consider myself pretty hand with carpentry work. At any rate we recently purchased a jayco kiwi hybrid camper. Overall the camper seems in good shape. But, I did notice the one corner above the pop out was a bit soft. So yeasterday I pulled it out. The contact board was wet I removed all that. The corner where the ply wood was wet so I removed that. I went further to the ceiling and there was a small section that was rotted so I cut it all out the remainder of the ceiling ply wood where I pulled back the laminate was dry and strong. So I’m trying to figure out how to replace this. I don’t want to remove the whole section of the ceiling. Just replace the area I cut out. I was thinking of cutting a section of plywood and using liquid nail to the styrofoam and putting new ceiling paper on? Unless I should try something else ? I did notice the water seems to be leaking from the aluminum bar that attaches on the roof so I plan on ceiling that up well tommorow. Like I said I wasn’t sure where to post this but there isn’t to many rv forums. Thanks