I wouldn't rule out the possibility that the converter/charger isn't working properly, especially if a WFCO and you may want to keep an eye on it with a voltmeter.
The first battery in our current TT lasted only a few months into the first season. Replaced it with the identical brand/type. Then that one died in the 2nd season of use plus the converter died at the same time (smelled of burning plastic + breaker tripped). Upgraded to an AGM and a Progressive Dynamics (retrofit) converter/charger. I have to think the converter was killing the batteries all along. From the very beginning, the battery never seemed to hold a charge properly and just put it down to phantom loads draining it (always made sure cells were topped up too).