More than likely the carbs are dirty, needles, seats and o-rings are shot due to the crap fuel we have today. Additionally, the 500 and 600 twins are jetted pig rich to start - you can drop your mains, pilots, and your needles until you're just right for the temps you ride in most often.
People have just accepted that the twins were pigs and that's just how it is..it's not. You can get these sleds to run 11-14mpg all day if you just do a little work..especially if the sled isn't modded..
Another thing to consider are worn out clutch weights and rollers..being a sled that's 15 years old there could be a whole bunch of things hurting the mpg..
My '95 that is piped and clutched was getting 11-12 mpg last week when I was riding in temps that were in the high 30's, when it's nice n' cold I'll see 12-12.5mpg. Used to get up to 13-14mpg before I dropped some weight out of the primary..point is, these twins can get decent mileage if you tinker with them and find a jetting and clutching combo that works good for ya..
Or, get a new sled and give the 700 to the gf!
Good luck!