What is the position/title of trustee within the Fraternal Order of Police?

The title is TRUSTEE. An elected position.

They are behind, in authority, the president, the VPs, the secretary and the treasurer.

All elected positions.

Trustees represent the rank and file if that FOP lodge is a representative union. They normally come from the unit representative (who is akin to a shop steward)

Most FOP lodges are not unions that can negotiate contracts but just fraternal organization of cops, like rotary or elks.

Basically it would be: (depending upon the City and By-Laws)

There shall be an Executive Board composed of the President, as Chairman, one or more of the first and second vice-presidents, as Vice-Chairman, the Secretary, the Treasurer, three (3) Trustees, the Conductor, the Chaplain, and one (1) Guard, and the Past Presidents, which shall conduct the business and the affairs of the Lodge between meetings of the General Membership to which it shall regularly report for approval or rejection of its actions.