Sadly no one has really answered the OPs original question on DIY travel trailer forums. The only one I have ever found is strictly for "tiny trailers".
If you are serious, I suggest searching the internet for a small utility/enclosed ALUMINUM trailer manufacturer. It will be expensive, because it will be all/mostly aluminum, but customized with a wheel chair lift or maybe just ramps, it would be a great starting place.
Flat aluminum sides do not look as "pretty" as fiberglass, but if you glue foam board insulation to the interior, it will take out a lot of the "wavyness". I don't think you can (cost effectively) replace the rivets with other means of adhering the aluminum skin.
Besides spending extra for a (mostly) aluminum chassis and body, another place to spend extra on is the floor. Do NOT use a standard 3/4" plywood floor. It is heavy and will rot over time. You want a 3/4" honeycomb, fiberglass floor that has no organic (wood) in it. Install vinyl "plank" over it.
Personally, I would go with a dry/composting (or cassette) toilet. No black tank to empty and less fresh water usage.
