Hi
i am answering this because no one has so far.I am not expert so this is just my thought (which could be wrong).
If your main duties fit 7312, and you can back it up with a letter from your employer, etc, you should apply under 7312 (if that is in the ministerial list). You should also send a letter explaining everything.
Do you have any other experience (in that last 10 years) that may fit in the ministerial job list?
To be considered a heavy duty mechanic, you are going to have to show that you had 3-5 years apprenticeship before you became a mechanic. Here is what the NOC states.
Employment requirements
* Completion of secondary school is usually required.
* Completion of a three- to five-year apprenticeship program or A combination of over four years of work experience in the trade and some high school, college or industry courses in heavy equipment repair is usually required to be eligible for trade certification.
PMM