Just leave it alone. This isn't a new Lamborghini we're talking about. It's a house on wheels built with cheap wood and plastic, all held together with staples and wood screws.
The interior shower garnish normally does not have a seal of any kind. It is held in place by a series of screws right up into the ceiling panel. It is not entirely unusual to get some condensation between the exterior shower dome and the interior garnish. No big deal. If the luan has decayed 1/4" in 8 years, the rig will be 32 years old by the time the decay has reached 1" and by that time, the luan around the opening in the shower ceiling is going to be the least of your concerns. I don't think I would even give it a passing thought.
The exterior shower dome is set in place on the roof with a bead of Dicor and then screwed down with a dozen or more screws, and then Dicor over the screw heads. Some manufacturers will use grey putty tape to effect the seal and then top it off with Dicor after screwing it down. Either way, that's how it's done and there is no reason to expect that the area between the outer dome and inner garnish is going to be airtight.
