OK this is driving my crazy so looking for additional input

I was getting about 18-19 mpg for the last 4 years, about 8 months ago
it just dropped to 13. I have completely gone through and can't figure it out.

other then milage, the only indication is a check engine light. but that checks
as an O2 sensor, which has been replaced twice with OEM.

Engine seems to run fine other then that,

anybody have any ideas?

Have you checked your connections to the "Bad O2" sensor with a v-meter? If the C-engine light is still on then you still have a problem. When O2 sensors go out gas mileage goes significantly due to the fact that your vehicles computer thinks the car is running lean and as a result it increases the gasoline flow.

Any other symptoms?

I would agree if not. Solve the check engine light first then worry about fuel economy if it doesn't change.