Ok, I finally got around to checking this out. I actually removed the first step and that allowed me to see everything I needed. There is no gray water tank, so there is a lot space in there. I took many photos:
The step removed:
Bigger pic here
Here's the camera inserted in the step facing right in the above pic, toward the front of the boat and the galley. The coils are behind the fridge. You can see the dampening material my dealer used to cut down on vibration (another problem):
Bigger pic here
Same view, looking up showing the inside of the forward part of the cabinet:
Bigger pic here
This shot is directly below the breaker panel, trash can removed. You can see the existing duct the blows above the galley cabinet immediately right of the breaker panel.
Bigger pic here
Same view as aboive, camera inserted in the space:
Bigger pic here
Same spot looking down, this is the trash can door below the breaker panel. The two images above are this same spot looking left. Lots of room to cut a hole and access below the step, which is all under this false floor.
Bigger pic here
My proposal is to T off the existing line in the above photos, cut a hole through the false floor above, run the duct in the empty space behind the fridge in the first few images, up into the small storage cabinet under the microwave:
Bigger pic here
The vent would mount behind where the clip-on fan is located in this photo, only sacrificing a small amount of space in the cabinet (door openin this pic):
Bigger pic here
Here is a movie of what I'm thinking. The quality is greatly diminished, but with the above images you'll get the idea. Ignore my narration.

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SOLD! 2006 30th Anniversary Vista 278
"Conch'd Out!"
http://vista278.com