If 12V, Lifeline batteries are very popular as AGMs, but pricey. Others have used Optimas, technically I don't think they are AGMs. If 6V, it seems like Trojan is the market leader on quality 6V batteries. If your current battery bank is vented and okay for lead acid and cost is a factor, you might consider maintenance free lead acid batteries.
Whatever you choose, you should be able to get the same size batteries you currently have. Make sure you get deep cycle batteries.
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Prevost XL 45 Conversion