97 camry went from 34mpg down to 18mpg and the fuel gauge stopped working correctly

Fuelly .com "Based on data from 144 vehicles, 6,414 fuel-ups and 1,892,793 miles of driving, the 1997 Toyota Camry gets a combined Avg MPG of 23.05
with a 0.15 MPG margin of error." 34 is hard to get today. Are you sure of that mileage? Bad sending unit or grounded wire would cause fuel gauge to not show anything.

I'm not sure where you're going with this but my daily driver is a 1998 Camry LE 2.4L A/T with approx 270,000 miles on the odometer and I believe my average is currently 27/28 mpg. I do mostly local/city driving and I'm not known to be gentle with the throttle.

Part of proper maintenance is having it correctly diagnosed, and addressing what needs fixing. Find a good mechanic at a good shop.