Contact the department of motor vehicles.. Vehicle repair shops are regulated by the state.. IF they are in fact "Running up the bill" then it's a bit like aircraft.. (The joke is when the weight of the paperwork exceeds the weight of the air frame it will fly) In this case when the weight of the letters of complaint exceed the weight of the investigator. it'll get investigated.
I know of dealers (Ford in this case) who have been padlocked by the state.
(One of the reasons I'm not a ford fan).
Also contact Cummins
It is possible they may want to look into their representative dealer. Their investigators are "lighter" re the joke above.