If this is the original factory tank Dave is talking about then it is plastic "Bladder" that fits into a well under the dinette floor.
What a bizarre idea they had with that one.
As I understand it when filled to the top the tank would lift the floor covering in that area.
It would be a simple mod to stick a normal tank in the underseat storage on the kitchen side of the trailer.
That is certainly what I have thought of and what I would do.