Are the batteries no longer holding a charge? If you are not observing real-world performance issues then the "damage" is theoretical at this point - so keep using them and see how it goes. could be over thinking this and they may be just fine, not like they sat for 9 months @ 10v
If you are observing real-world performance issues with the batteries then you have a few choices:
(1) Ask the dealer to replace under warrantee and pretend you don't know what happened (dishonest on your part)
(2) Tell the dealer what happened, and let them try to replace under warrantee (dishonest on their part, as no manufacture will warranty if product was abused)
(3) Leave the dealer out of it, write off the batteries if they are truly underperforming, drop new ones in yourself (no need for dealer to help here, they are not battery experts).
Honestly, I highly doubt the dealer is going to provide any good will credit / savings over $200 worth of batteries. No margin / profit in it for them.
We all make mistakes - it's a learning curve with these trailers. Having been there, I chose option 3. Lots of good threads on this forum on what best to replace with (6v vs
12v, flooded vs AGM vs lithium, etc.).
This may be your excuse to upgrade to some lithiums with have battery management systems that cut out automatically to prevent damage due to low cell voltage.