After buying my 2002 Coachmen Pathfinder in March I found that the manual was almost worthless. The RV had had at least 3-4 previous owners who all did different mods, some of which still worked, many of which didn't. The manual didn't have any of the REAL details like how to bypass the hot water, how the 12V system worked, etc.
Trial and error. If you don't know what something does, google it or ask here. It's taken me 2 months and several trips to get a handle on how my RV really works, but I think I understand 90% of it now. =)