If Roadtrek is using the same plug now as then - and I believe that they do, this is a Roadtrek specific part. As described above it is just a plastic plug - like an upside down cap with a little tab inside to grab it with that pushes into the hole with a rim so that it cannot be pushed into the tank - and it sits flush with the door frame so that the door can close. And as described above there is a little air hole in the bottom.
Some Roadtrek dealers with parts/accessories shops carry them in inventory. I bought two spares from Fretz RV in Souderton, PA. They do mail order and may be able to tell you if they are the same as for the 96. If they don't have it on hand they can order if for you from Roadtrek. Any RT dealer can order it from Roadtrek. RT customer service on the RT website should also be able to tell you.