Early Trillium design in front was much different than later models.
In 1979, they switched from a solid gaucho across the front, with no gap for a table, to a gaucho with a cut out in the middle. At that point, removing the bunk and switching to a dinette table was easy. Prior to that, you are cutting out a large portion of the gaucho bench, which is fiberglassed to the floor and front wall. So its a serious undertaking and IMHO, the gaucho does help the trailer structurally hold its shape side to side.
One thing to consider on the interior of the 1300 and 4500 Trilliums is the interior cabinets are fiberglassed in place to the fiberglass floor and the fiberglass outer walls (not held by rivets like some brands). Most have multiple ribs in them for further strengthening. Ever notice the vertical rib in the kitchen backsplash? There are similar vertical ribs in the dinette benches and the front gaucho. All of this allowed Trillium to make the shell thin.
I don't think the original designers are still around. It would be interesting to hear their reasoning in the design. We may be giving them too much credit, or not enough.
A picture of my 1977, you can see the vertical rib in the lower kitchen cabinet and the center of the gaucho. You certainly are not just removing a bunk and dropping a table in place. You are doing some serious fiberglass modification. I do not consider these ribs to be a cosmetic touch, instead, I believe they add significant support. When they later replaced the full across the front gaucho base with individual benches, they created additional strength by adding a corner to each base and having the fiberglass continue forward to the front wall where it is attached.
But I will leave that up to the fiberglass engineers to tell us for sure.
1977 Trillium Painted Cabinet Doors by
wrk101, on Flickr