2 2015 GU815's having starter problems

Hey guys,
We have 2 2015 Mack GU815's, both purchased new at the same time, both trucks in the last few months have been having an issue where the starter solenoid just clicks and wont turn over for several attempts. Batteries are at full charge, starters have been changed, wiring has been checked, one shop found a ground cable had worn through on the frame, but this did not affect the issue. Changing starters reduced the frequency of the issue.

Mack has no idea how to solve it and has suggested it could be the neutral switch on the transmission and to contact Allison.

Anyone else had this issue and found a solution?