I ran into a similar, but smaller, problem with U-Haul on a truck rental a few years ago. They rented me a piece of junk, which broke down, and they tried to charge me $900 for towing and for the wages of the drivers who drove it back. (note the conflicting lies- they both claimed they towed it and they drove it). It took about six or seven months of letters back and forth with them and Amex, but in the end that conflicting lie became blatant to Amex, who ruled in my favor and cancelled all charges.

Document EVERYTHING and be insistant with your credit card company; contact any AU agencies similar to the better business bureau, and any regulatory agencies which oversee them. You can also report the dangerous condition of the vehicle to the local department of transportation, try to get them to do a surprise insection of this companies vehicles. Documentation of their unsafe conditions will be helpful.

Best of luck, mine was a huge headache over $900 U.S., and yours is much larger.