P0446,emissions;a perpetual problem. no one can fix. is it a guessing game?

Thank you for your fast response, sorry to burden you with a long reply; I am in need of good advise. Should I ask the Authorized Serv. Center (David Wescott) how they determined the canister was the fault. First they said it was the evap vent valve, I told them it had just been replaced by Tire King($440.00). Then they said, I had to many miles on the truck since the check engine light came on. They reset it and told me to come back when the light comes back on, which I did. Then they replaced the Charcoal Canister ($500.00). Two days later, light back on. They said bring it back next week and we'll do another diagnostic evaluation. ($70.00). I think it would be best to stick with one person until they get it right, but I'm not sure they will not charge me again. Should I ask them if they know how to test it to find the problem opposed to trial and error? This truck has had the check engine light on eight times in one & a half years. 1. O2 sensor (mechanic fixed) had to take it back twice, 2. I changed the plugs, 3. I changed the plug coil and 4. Gas Cap 5. I changed the Thermostat 6. Evap vent valve, Tire King worked on it. 7. Evap vent valve, David Wescott worked on it. 8. Evap vent valve, needs to be checked again. David Wescott, Serv. Center said the computer showed that someone has been into the engine of the truck. Not while I've had it (since 2008). Is all this typical for this vehicle with 87k miles? or is this an indication of on going problems for the life of the truck? Thank you, Cecelia