<VV> Saturn help? No Corvair

[Grant]

Knowing that Corvair owners are more apt to own other unique cars than most, I am reaching out to any Saturn owners with a similar experience or knowledge. A future grandson-in-law has a 2007 Aura (hybrid) whose mileage has recently dropped from the high 20's to the 10-12 mpg range. There are no codes set and only a slight rotten egg smell right after a fill up that quickly dissipates. We can't find any fuel leaks or smell the converter when idling. There has also been a slight drop in performance as a lag under acceleration. Any hints on where to look or check would be appreciated. Selling is not an option at this point even at 130K.
Thanks
Grant