Re: 1989 Hunter 40 - A million questions.
Do you have any idea of the age of all the diesel hoses? Are they really made for diesel? It is very popular to use cheap hose, looking rather like high-quality diesel hose. These rottens from inside, leaks are very difficult to see (but easy to smell, as you know).
In particular the filling hose, and the vent hose could easily be of this simplier kind.
Otherwise I would agree that the return hose are suspect #1.
BTW: how did it go with the water tank?
(another disadvantage with one thread for all issues ...)