Thank you idliman - some really useful suggestions. My inline comments are given below:
1) Prove that you worked in the Bank for 5 years (by providing paystubs or W-2's)
=> This is not needed as the letter from employer confirms start and end date (but not job duties)
2) Show the PERM job description from 9089.
=> Great suggestion! Unfortunately, Job description in 9089 does not cover my managerial responsibilities, which may become an issue as PERM requires 2 years of management experience

I was able to find a letter that the employer wrote to USCIS in support of my H1B extension (ongoing job). Again, this does not describe my managerial responsibilities, focuses more on domain expertise and technical skills. However, this may prove to be helpful - will include it in RFE reply.
3) Usually attorneys include descriptions in I-140 application. You can use that too.
=> Same as 2, I-140 has the attached 9089.
4) Instead of getting experience letters from one co-worker, get it from 3 or more. Tell them that USCIS may call anytime to verify.
=> I am able to get a notarized letter from the person who was my direct supervisor for 1 year. Going to add this to the RFE reply as well.