Unless OP has photos (always a good idea in iffy situations which OP acknowledges), this is a he said-she said. OP has attempted to resolve the issue and the best he can now do is to initiate a chargeback. Car rental companies document their stuff well, so it's a likely loser. But, OP can try.

There is always the risk with Avis -- and other merchants -- that if the chargeback is upheld, that Avis simply shows a debt and DNRs (Do Not Rent) OP. That may or may not be a big deal.

A final word of advice. If you've put the vehicle through the wringer, it's well worth the $10 to run it through the car wash and vacum it out. It's appearances that count.