Yea, you should be doing better on fuel. But I don't know the AT ... just basing on that engine, which Honda used in several Hondas we got here in USA in that era ... we never got the AT but did get various cruisers and the TransAlp which used a similar (or same?) 650cc, 3 valve V-twin engine, IIRC.
25 to 30 MPG is piss poor mate. I believe your motor is a 3 valve motor, right? Must you adjust valve clearance on those or are they hydraulically adjusted?
I would go through everything, system by system. If the bike is new to you, then you either must contact previous owner and find out exactly what mods he made and dig deep to make him reveal the "Real" problems he didn't tell you about. How many miles? Or, dig in yourself and have a close look at .... everything.
1. check to see if electronics are original. Test them all. Consider replacing any weak bits with NEW OEM Honda items. (coils, CDI)
2. Install new, ND Iridium spark plugs.
3. Check air filter. If standard issue, consider a more free flowing one from the aftermarket. Make sure Air Box is CLEAN and unrestricted.
4. Pull Carbs off bike, strip and throughly clean them. Check jetting. Main Jet,
Pilot or any others in there. (I'm assuming they are CV Carbs, yes?) To start, go back to original jet sizes if you find non standard jets in your Carbs.
5. Inspect exhaust/mufflers/headers for any leaks, restrictions or modifications. Consider an after market system ... if you can find one. Freer flowing exhaust will not only improve performance overall, but will also improve fuel economy.
6. Do a leak down type compression test. This one is a biggy, very important. If the valves are worn out or rings (less likely) then fuel economy will never be good ... even though bike runs OK. But compare with a truly "correct" same bike, I think you will see a BIG difference.
7. Oil Use. Have you tracked it yet? If using oil then fuel use will GO UP too!
Change oil, but use a good quality crankcase flush to clean out crank case before adding fresh oil and filter.
Then go to best quality synthetic oil you can afford. I like Mobil One. (bike specific)
8. Check valve clearance (do this FIRST!) Once set, monitor for next 5000 miles, make sure they stay in spec. If they don't, you have a more serious (but fixable) problem.
You could do all this your self if you have the skills and deep knowledge of the AT ... or entrust your AT to an AT Guru and see how it goes. These old gems, IMO, are worth fixing up. Once good and restored properly, should go trouble free for years ... BUT NOTHING LASTS FOREVER WITHOUT TENDER LOVING MAINTENANCE!