You might post in the Monaco/HR owners forum on this site - there are probably other owners who can help you out. Your Navigator, though, is rather different than other HR models of that vintage, so don't get frustrated if you get advice that doesn't seem to apply. For example, I think yours is the only HR model with basement air in that time frame.
The HWH reservoir shouldn't be hard to find - just follow a jack hydraulic hose back to it. The valves are usually clustered on the reservoir and I would expect it to be mounted in the back on a pusher chassis like yours. Try the right rear side.
I don't want to run you off, but there are a couple active Holiday Ramblers Owners groups on Yahoo Groups . They are the ramblin_pushers and the HRmotorhomes groups.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL