Roger, Most of these repairs can be done from the inside. Best just to start taking it apart. These things were built in modules so when something has to come out, more than likely something else is in the way and has to be removed first. Plan on ripping lumber to odd sizes. Remember Google is your friend. Always start with RV.
Like you said it was free, don't go overboard. If your routes take in N. Indiana, there are a lot of RV surplus places, look for RV bone yards. Most things are interchangeable with minor mods. AC units are sized by BTU's and 4 bolts clamp them to the roof through a 14"x14" hole. Refitting RV plumbing is a nightmare. The quick easy one time fix I'd use a sharkbite fitting, costly. Get a pex tool and some line, once you have the tool it's easy.
Your beginning a new education. No more trouble than getting your ticket, just a different skill set.
73's KA9ATV