I have two AGM's. They are not as good as the people who try and sell them to you. Stick to Walmart 12 volt $99 deep cycle. I spent $250 per battery and they do not have the power/amps as a Walmart deep cycle. Wasted money. Yes there is new battery technology out there. But it is not AGM's. AGM,s only means they will outlast a conventunaly battery. Yes, as an engineer, what they are saying, it will outlast your conventional battery, but as a cost of power you need to run your coach. You can google Lithium but open your wallets. I live off the grid...... 24/7
Dont know about AGM's 'lasting longer'... But thats not 'only' what you get...
-no checking water, or more important the procedure to do so.
-no corrosion on the battery posts, giving more power through the connection longer
- VRLA batteries will self-discharge less than 3% per month at 77º F (25º C). Flooded batteries will self-discharge up to 15% per month at 77º F (25º C). (Per Trojan) For those of us who might be grid suckers, and neglect our batteries for ...lets say 3 months

.. it means the batteries are at ~90%, not ~55% after this time passes. More power for the coach, longer battery life.
Yes, the power/amps are a little less. My 12V group 31's are rated 105AH. I found this FLA group 31 (
https://www.homedepot.com/p/Nautilus-31-Deep-Cycle-Marine-Battery-31MDC/205309650) and its 115AH. 10% less capacity in the AGM. I consider that a good tradeoff for the benefits listed above!
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A bit of empirical data.. I have (4) gel cell batteries dated 6/16, (2) trojan AGM group 31's dated 5/18 and 3 group 75 FLA gas cart batteries dated 12/17-5/18 on my spares shelf now. For a few months. I check them weekly with a voltmeter, and supply a charge as necessary. The FLA's need charge monthly, they fall to ~12.2- 12.0 volts sitting for a month. The AGM's get charged every 2 months so far, they tend to go down to 12.6-12.7 after this period.
The gel cells are impressive. Fully charged at new years 2019, they are STILL at 13.03-13.08 (!) rest voltage. 98AH, so not a big capacity penalty. I salvaged them from a Gem Cart, there was a receipt for $2200 for (4) of them dated 8/16. Whoa! The owner didnt know there were 2 more up front..

and never asked before selling the cart to me as broken. Cheap! Ripped the batteries out and sold the rest for what I paid.