Please Help- must travel far- 14 mpg!!

Okay guys here it is- I am looking for any advice about my issue before I suck it up, grab my ankles, and bend over for Toyota. I have changed my O2 sensor and A/F Sensor (Before Cats) to get my check engine light to go off- The rear sensor is a Bosch and the front is from Toyota. Upon changing the rear sensor light did not go off and changed the front and it did. I thought the problem was solved until my gas mileage dropped about 7 or 8 miles per gallon! I tried changing the plugs- they all looked ok, I tried seafoam (truck seems to idle a little better), and I cleaned my MAF Sensor and throttle body-- No change....I scratched my head and then remembered another post that talked about "soft codes" the computer stores so I travel to Advance. The guy hooked it up and I pulled 3 codes on the scanner but have no CEL light??? they are....

P0440...EVAP incorrect purge flow
P0441...EVAP VSV Circuit fault
P0446...EVAP vent control malfunction

I have not idea what these mean, nor am i sure that they are related to the drop in MPG that happened. The only stupid thing I did was hack the old front sensor to death trying to get it out- Ohio has bad rust problems, basically cut it in half leaving an exposed hole in the exhaust and then drove it to a buddys house about 30 miles away to use a torch to free the bolts. Then installed sensor. I noticed the MPG drop on the way to his house- you could and still can watch the gas gauge needle go down. I am beyond my knowledge here??? Other than that the truck idles fine, drives fine, no funny smells, no huge loss in power maybe very minor, and sounds ok. Toyota wants $102 flat fee just to look at it and maybe tell me what the problem is and judging by the guy I talked to on the phone (who didn't know that the computer could store codes without throwing a CEL light) I think the minds on this forum are far superior. Any help would be greatly appreciated.....I have to drive to Tennessee next week which is 8 hrs away and 12-14 mpg won't do my wallet any favors.