Why the drop in gas mileage and occasional awkward shifting?

Bout a month ago, my mom's 2006 Honda CRV randomly shifted
awkwardly from reverse to drive (or drive to reverse, whichever) and the
car shut off. It started up fine except now it sounds slightly louder than
normal and her typical 23 miles per gallon is an atrocious 15 mpg. She
changed the typical things like air filter, spark plugs and added fuel injector.
There was only a total 8 mile increase for the entire tank, which is nothing.
No mechanic has been able to diagnose an issue because the check
engine light is not on. They all say it sounds and runs fine, therefore have
no reasoning for the drop in mpg. But there has to be, tire pressure is
normal and driving habits have not changed. What could cause mileage to
drop this much? Is it fixable?