Often such information is not available (or at least very, very hard to come by) for RVs even if the manufacturer is still in business. Looking and poking around is often the best or only way to figure things out. Generally, the individual systems are not too overly complex if you look at them in isolation from each other. (If you try to understand everything together all at the same time, as a single system, it can be very daunting indeed; so I recommend approaching things as individual systems and pieces.)
If you need chassis information, that should be available from Ford or the usual other companies (Chiltons, Haynes, etc.). Since the E series vans were very popular, there are also lots of YouTube videos, forum posts, and so forth online about most any chassis maintenance thing you'd run into that can be found with a bit of searching.
Finding service documentation on individual appliances and other third-party bits is often not too hard once you get the maker, model number, etc. from the thing in question.
