Of all the RV manuals I've ever seen, none of them has ever been as good as a manual that comes with a typical car or pickup truck. Most, if not all, of the manuals I've seen have been so generic that not having the manual wouldn't have been a significant loss.
That being said, you don't have any manual, so you are kind of on your own. My experience with Class As of this vintage is that fresh water tank fill is gravity fill, like a trailer, and you'll find your fresh water tank fill in a locked box on the side of the vehicle. I realize that isn't your issue, but, let's work through this by process of elimination. Once you have found that, the fresh water tank fill port, if the city water/park hookup isn't there, on to another compartment or hatch. This is where just walking around the coach looking for ports or access points is really important. Is there another small hatch on the side of the coach that might be key locked which you haven't explored. Maybe that is the city water hookup. The key here is to walk around your RV closely inspecting the sides of it for access ports which may be obscured by striping or paint jobs.
Again, general observation of Class As of this vintage, the wet bay, service compartment, or whatever terminology you wish to use to call it, should have most, if not all of the park service and sewer dump items all in one place. I would be very surprised if your city water hookup wasn't in the service compartment, but I think it unlikley. So, find the location of your sewer tube connection and/or your shore power connection and I bet the city water hookup will be nearby. Again, this may require some walking around the coach with all the basement doors open looking inside each basement compartment for anything jutting out of the walls or ceiling of the compartment. Also look for tells. Most service/wet compartments in the basement will have some kind of false floor or pipe/wire pass-thrus to make park hookups more elegant and allow the compartment door to close fully and even lock while wiring and hoses are connected to the rig. Did you find a basement compartment that basically is just a basement, but there happens to be a hole or access port through the floor of the compartment? Your city water hookup might be in that compartment and the pass-thru in the bottom of the compartment is where you are to route your freshwater hose.
A lot of the "getting to know you" phase of buying and using a used RV is figuring out where everything is, usually be chance and experiment. Sorry I couldn't just say "look here, it's there", but I think most used RV buyers go through this process for a few weeks after purchase before the RV becomes as comfortable to them as it was to the previous owner.
Happy camping!
