Ya.. how about a hardwood 2x2 or 2x3 under the seam. If attached the length of the one cabinet end wall would cut the span back down. You'd have to butt to the other wall but could put a small trim block on wall to hold up.

Id probably glue to ceiling also but either way would minmize downward movment of just one side of the seam verse the other (and support at back of the vent).
Being careful when up there, could wait till done- me Id be cracking the roof. Id still put a temporary 2x4 with a block or piece of ply under seam/vent until done. I just saw it..seems you have too.
Im sure it was a lot stronger when new & one contiguous piece but still suprised your camper came with a ladder:).
I dont believe glue will react oddly with the seam tape-it may not want to dry so on it I would tend to use a bit less glue.
Only other suggestion is on the front older ply may be same issue, glue slow to dry as its already semi sealed from old glue? When you start putting stuff together especialy ac, vents etc, where your on your knees & toes is using some clean cardboard or foam on roof to help keep from compressing spots. I know i got on mine a bit too soon and left foot prints, one area the rubber was slipping and bunched where my boot toes were digging. It takes it a while to actually cure.