The fill should be where you looked
I have a 1985 H31. On it, the FW fill is indeed where you looked, as is the access port for the emergency tiller. Both are under aft most teak plate just in front of the swim ladder. I am positive that is the factory location.
You can look in the boat info section on this site and get a link to a manual, but it wont tell you what you need to know. Even if it did it would direct you to where you have already looked. It will give you plumbing and electrial diagrams however.
If your fill port is not under that teak panel I would suspect the boat was modified. Is there an empty 2" hole under the teak panel? Does it look like a 2" hole was glassed in?
There has to be a fill hose going to the fresh water tank someplace (assuming it was relocated and not just removed). Perhaps you can find where it was moved to by tracing backward from the tank. Do you still have a water pump on board (in the deep cockpit lazerette). Also the hose bib for an external water connection is in the cockpit on the starboard side. Is that still there?
I attached the link for the manual pages for you