Re: Venting the fridge out the side: This will compromise the performance of the fridge AND require you to cut a new hole in the skin, since the fridge compartment door on a 24' Argosy is just the small lower door. One thing I'm considering for my '76 (which currently has a dorm fridge) is a Dometic Tundra 12v
Danfoss-compressor fridge that doesn't need all the venting. They cost about the same as a 2-way unit of the same size, but if you boondock a lot you'll need to add battery capacity and solar or a generator, so it's not a perfect solution either. Another advantage of the Tundra is improved performance in hot weather compared to an absorption fridge.
Design the dinette carefully. One of the frustrations we have in our '75 (which is a 24' Argosy with a front dinette) is that it's inconvenient to get in/out of the street-side seat of the dinette because the galley cabinets are in the way. If you're skinny gymnasts or have small kids you can seat on that side you can get around that, but I was specifically looking for a trailer with a gaucho and folding table for the next one. The dinette is nice when we're sitting down to dinner inside the trailer, and at all other times we'd prefer a couch.
More food for thought on your dinette design... we have exactly the same water tank in the '75 with dinette as the '76 with gaucho. That leaves room for storage under each dinette seat, but it does compromise the legroom under the table. If you wanted to seat 4 at the dinette, it would also be useful to have a kid or two for sitting close to the front window.