I installed living quarters in a 32 foot cargo trailer a few years back. Complete everything, plumbing, electrical, sewer ,water lines. There is a book that explains 12 volt systems and also the tech section on this forum was a valuable tool . It is quite an undertaking you have in mind but is doable with proper planning. If you have access to the plumbing and wiring I would recommend replacing each item you have in front of you as needed. Fitting paneling to walls and trim moldings require patience . Use templates out of cardboard before you cut anything. Save pieces you take off to use as templates.I assume you have basic mechanics experience and use of hand tools safely.