These batteries are designed for engine starting applications. They are not recommended or warranted for use in deep cycle applications.
Based solely on what you have posted a bigger alternator was what solved that problem.When I first got my rig, with all acces. on I could watch the battery voltage drop as I sat in traffic. 40 amp alty couldn't keep up. Since then I've got a big honkin battery and a 90amp alty. Problem solved. but only a bigger battery nor only a bigger alty wouldn't have solved that issue.
That's what the bigger battery gives you.I went with a marine deep cycle because I knew I would be camping with the radio blasting for hours on end.
At idle, the stock alternator WILL charge the batteries with the stock accessories on, so if your larger alternator won't, then it's possible that you're not using the best alternator for the job.BUT even my bigger alty, at idle and all STOCK accessories on does not supply enough current to charge the battery