To begin with, congrats on purchasing your new boat.
Having owned several of the 300 series Rangers over the years I can tell you that getting your hands on any owners manual would probably suffice. Those boats like most others only have three to four components. The first is the hull, followed by the electrical system, plumbing system(which you're having problems with), and then all the goodies we attached to those boats (i.e. engines trolling motors, and electronics).
I doubt that Ranger is going to maintain an inventory of parts or manuals on a boat that's 20 years old when you consider the hundreds of models they've produced over the years. Fortunately, there are just as many if not more fishermen who have owned these boats, and the only real differences of all the 300 series was limited to length and the layout of the cap. If you have a specific question, the odds are pretty darn good that you'll find someone who has your answer.