What can be my problem? I have changed my spark plugs, wires, pcv valve, and the coils. Whats next?

A bad oxygen sensor is often the source of bad milage. If you can, change it out with a OEM replacement, stay away from Bosch O2 sensors seems to be a consensus here. Good Luck.

i get about 18mph with a mix and 22 or 23 on the highway that was also with the air on

Where is a good place to buy 02 sensors??? The cheapes I have found is $70 for Denso ones. Also is there any way to tell that the o2 sensor is bad before replacing it?

The only way to tell is with a scantool. O2 sensors usually are starting to get tired by about 50-60k miles. If you don't know when it was changed last, you should do it. No gaurantee it'll fix your problem, but it can be a major contributor. Buy ONLY AC Delco. Part numbers are in Techinfo.

maybe its bad gas, i know i used to buy gas at cenex and now i only buy at shell and i've noticed about 4 mpg change for the better, it'* deffinetly a good idea to try.