I've ofter recommended plugging into a 20A GFCI to make sure there are no ground faults. Testing and isolation will likely take some time. It will probably be tempting to replace the CB. But keep in mind that the ground is a safety ground and is not suppose to be carrying any current as that is the job for the neutral.
If you don't fix the rig then what is the risk? Let's say you plug into one of all to many pedestals with a open ground - bingo your rig frame now has 120V which can be quite dangerous.
While we can step you through some testing your posts suggest that you should hire the electrician that said he would have to test each circuit as he appears to understand the problem.