Need help in writing a thank you letter for all my
Union brothers and sisters who elected me as Recording
Secretary for our union

Since, I suspect, the vote was by secret ballot, you can't thank individuals and you might be antagonizing those who didn't vote for you. So I would just make an announcement that can be entered into the minutes at the meeting where you are invested in the position to the effect that you appreciate the confidence of those that voted for you and plan to do the best job you can.

By the way, please take more care when choosing where to post here. I moved this from the Bereavement forum to a more appropriate forum.