Again, I agree with Tech. What you describe sounds more like an issue with the converter/charger....which ruined the battery.
Warranties are frustrating. It seems everyone wants to point the finger at someone else.
I am not a lawyer or legal expert but, it seems if the RV dealer provides Interstate batteries, they should be an authorized "Interstate battery dealer". If so, the RV dealer should have replaced the battery and worked out reimbursement with Interstate. For example - the dealer didn't manufacture your camper, but provides warranty service on it, and then, gets reimbursed by the manufacturer. Make sense???
Unfortunately, things don't always seem to want to go this way. Good luck!