Fuel tanks cracking

I have a 2000 freightliner classic xl with 140 gallon tanks I have suddenly started having problems with the welds cracking around the plate that my fuel lines attach to. I tried having it weld with no luck. So I ordered a new tank, I put it on and 4 days later the other side cracked. I ran for about 5 weeks on one tank trying to save money for another and the brand new tank cracked in nearly the exact same place. I have no idea what is causing this I put new rubber on the tank straps when i replaced the tank. I pull intermodel containers out of the rail and im on pavement 99.9% of the time. Any advice would be appreciated

 

This is the old tank manufactured 10/99

 

This is 10/16

 

There is slack in the lines no supports really the lines are all zip tied together up near the frame

 

Never