Now that time has passed, I can write up this experience without the blood boiling so much.
http://www.bartrak.com.au/ (a Melbourne agent for http://www.aamotorhomes.com.au) in Berwick are sharks. Beware if you use them: don't go near them without a print-out verifying *everything* you ordered. And DO NOT PAY until you have seen the vehicle you will get.
Our road trip holiday started out with a nightmare. bartrak.com.au Berwick outright lied to us: we booked a 6 berth automatic and when we got there, they told us they only had a 4 berth manual (we can't even drive a manual). When we complained that this isn't what we booked, the response was: can you prove it? They wouldn't refund us, or even refund us the difference between the 6 berth we paid for and the 4 berth automatic they begrudgingly made appear after two hours of fighting with them. When I rang their Sydney head office, the guy was rude and hung up on me. Since they already had our money and would not refund us, we took the 4 berth automatic.
Once on the road, everything was great. We forgot our troubles and began enjoying ourselves: the motorhome itself was pretty good. The people were not.