Check Engine Light ON

My 02 Tacoma has the check engine light on, error code 0440, evap error etc…
Had one mechanic replace evap canister, resent check engine light, sent me on my way. After 30 miles, check engine light back on. This mechanic doesn't want to fix anymore, too hard for him to diagnose problem. I bring Tacoma to dealer. Truck has been there for almost 2 weeks. The dealer has done "smoke tests", replaced the fuel filler tube, plus a number of other BS parts they keep throwing at it. Still nothing works. Now the dealer says that the gas tank is so corroded where the filler tube meets the tank, that a new gasket and silicone won't seal the leak and wants to replace the gas tank for $1000. Seriously? Has anyone else out there had their gas tank corroded and need to be replaced?
My truck has been garaged kept since day 1. What would cause my gas tank to be so corroded or is the dealer full of @#$%.???
I have over $1500 in repairs so far, just trying to get check engine light off, in order to pass the emissions test.
Help!!! I'm drowning in dealer BS!!!!

 

I've seen a few gas tanks like that. it will cause a leak and throw the CEL.

being from Chicago you are also privy to salt. even if it's garage kept unless you spray it off salt can still accumulate on top of the tank and cause corrosion

 

Have they checked to see if the gas cap is just cracked? I had a nissan that had a cracked cap that would start separating when you tightened it.

 

I have replaced gas cap with a "Genuine Toyota" gas cap

 

The gas tanks will corrode from the inside, Suzuki Samurai's are notorious for rusted tanks. You can thank the fuel blends.

 

This isn't helpful to getting rid of the check engine light but when I was in ga if you spent so much money to pass emissions they just skip the test and allow you to get new tags. I don't know how much you have to spend and it wouldn't solve the problem but it would give you time to figure it out atleast