Wow... :snorkel:
I wouldn't be so fast to jump on the AGM bandwagon like that... They & wet-cell batteries are not at all apples & oranges as they are Gala & Fuji or something more to that effect.
Unless you got a good specific reason to need a AGM battery (which there aren't very many), IMO I think a GOOD wet-cell battery would serve you just as well (which the DieHard RV battery is NOT, based on my experience with them some years ago).
A good wet-cell battery is the Delco Voyager. I've used these almost exclusively for decades, and every time have consistently gotten 8-10 years of great life from them. They've also never leaked fluid like I've seen with others such as the DH RVs (what also eventually leads to terminal corrosion, and no doubt is a common complaint with other brand batteries). Yet I have buddies that insist on blowing 2-3× as much coin on AGM batteries of various brands and almost invariably they don't last any longer or perform any better than my Voyagers do.
The Delco Voyager is made by East Penn (same company making many of the respected AGM cells these days), though I'm not sure if the Voyager comes in a group 31M or not (I don't recall they did last time I looked). The Voyager M27MF (what I have) is 105Ah (10Ah less than that DH RV, or 5 more than the DH P) so unless you're strictly tied to the 31M size, a Voyager 27MF I think would be worth looking at for a lot less $$$ (about $90 each 3 years ago last time I bought any).
Anyway, that's my take on it.
FWIW, the DH RVs I had were made in Mexico by Johnson Controls (who also makes the Optima AGM). I avoid all JC batteries like the plague.
Also FWIW,
I recall reading somewhere all of Carquest and Napa's house-brand batteries are now manufactured by East Penn, so maybe that could be something you can go on if you need a 31M (I've seen 31Ms at both places).
Good luck on it.