After reading your other post I take it you are talking about a 1995 F-150 5.0L and a 4R70W transmission.
For the canister purge solenoid valve failure code check your wiring at the canister purge solenoid valve and make sure that it is even there. I do not think that truck ever had an E4OD unless you put it in.

I would go with the $62 at advanced gas caps and no I do not think they are the cause of the canister purge solenoid valve failure code.

The bad gas mileage I take it was before you replaced the vacuum lines and if you did not have vacuum on the Fuel Pressure Regulator (FPR) your fuel pressure would have been too high to the Injectors and would give you the bad gas mileage. Make sure you have vacuum on the FPR now and recheck the MPG.