Believe you are correct regarding 10-6. F-Frank most likely is a code letter (Frank being the Phonetic for letter F, like ham for A = alpha etc). It designates either a location or a function like meal break.
Most likely, they're telling dispatch that they will be busy out of the car e.g. putting fuel (F) in the unit. They will change periodically just to keep all the scanner monitors guessing. Listen long enough and it's not too tough to decipher.
Seems to be strictly a localized code...not state-wide.