Toytaco, I have a 2001 Eagle that looks similar to your empty shell picture. I think your post was asking how others have configured their unit, and my mods have been minimal, but that’s partly because I’d rather be camping than working on it. I’ve owned it for 7 years and I think the best advice is to use it as-is for a few trips then start making mods based upon your desires and preferences. Over the years I’ve gravitated towards keeping the inside space as open and flexible as possible. The fold-out couch or any type of fixed furniture or table on the passenger side will use up a lot of space. In the Eagle, there’s not very much space in the aisle with that full-length couch but it is comfortable for relaxing and stretching out in cold and inclement weather. I take out my couch and support structure sometimes, and put it back when desired, as noted below.
I have a full galley on the drivers side which was the original equipment, and includes a 3-way refrig, furnace, sink, water tank, 2-burner stove, 20-lb horizontal tank propane box, and a storage drawer plus storage under the sink. The propane box is accessed from an outside hatch and it takes up a lot of the space directly under the stove. Let me know if you want me to post a picture. There's pros and cons to having it all on one side of the interior. The pro is that it’s all nicely contained on one side of the camper. The negative is all the weight is on the driver’s side.
I’m not sure if your inquiry was asking about the design of the original couch support, but here’s my explanation of it: I have the original equipment that I remove and put back in place as needed for a particular trip. It's not my preferred configuration and it currently is not installed in the camper so I can't post a picture. The couch sits on the edge of 1/2" plywood on 2 sides which basically forms a long box under the couch when connected to the side of the camper. Everything is connected with L-brackets. The plywood “box” is 12" high and 17" wide in the back where there is no wheel-well bump-out. It's also connected on the top with 2 or 3 pieces of the 1/2" plywood (6" x 17") that lay flat and are screwed into the wheel well bump and also connect to the vertical piece of plywood with L-brackets. Those pieces also form the support for the cushion.
The legs under the couch fold up when it’s pushed in as the box provides all the support. I think you know that when the couch is pulled out the legs need to be extended and the smaller vertical cushion needs to moved to a horizonal position next to the big cushion to create the full width of the single bed.
On some trips I prefer to carry bikes inside the camper instead of hanging them outside, thus the bed and support frame come out and get stacked in the garage. I’d really like to design some fold-up bench seats that are able to flip up into the space in front of the big side window. I haven’t been able to find a good hinge design yet. I currently have a Lagun table mount that I use with small individual bench seats but that set-up is my version 0.1 and is very rustic.
Overall, my objective is to live and cook outside the camper as much as possible but be able to utilize the interior space for sleeping and shelter when needed.