30mpg, with an automatic? Sounds low, but also doesn't sound like you are measuring your mpg all that well. I mean, you're saying you get around 300 miles on 10 gallons, not that you've been getting mid 30's or so, so I question your method of determining low mpg (not your intelligence, just your method).
Automatics get lower mpg, so that's good for like 5mpg or more. Driving at 75 is good for another 5ish. Lots of accelerating in city driving, stop and go, could be good for another 5mpg. Ergo, from 50 down 35.
At this age, though, you could have a clogged air filter, or a clogged snowscreen. Or maybe the fuel filter needs to be changed, but I think that would cause loss of power, not loss of economy.
Has the mpg's dropped recently, or has this been on going?
Can you do a few tanks of completely filling, and then calcuate the mpg's? It'd be best if you vented and used that, but if you do a few tanks at least one should be able to see what you are averaging, and then determine if it's really all that low for your driving habits.