Bad Interstate battery

I had a two month old Interstate battery boil out one cell. Noticed the sulfur smell.
I contacted the manufacturer (Grand Design). They won't cover under warranty because the dealer apparently puts the battery in. Interstate replied to my request for compensation with this:

"Thank you for contacting us.

Unfortunately, in this case, the warranty would not be applicable because that would be considered “failure to keep the battery properly maintained”.

It was a brand new battery when we left for the summer!
Sounds like poor manufacturing to me.
Thoughts?