Follow-up and (Hopefully) Final Post on 2016 Fleet Replacement Batteries
1) I pulled one of the old, dying Exide Edge AGM batteries and measured it: W 6 5/8" x L 10 1/4" x H 8 3/4" (H includes the terminal posts). So that is in fact pretty standard "Group 24," as far as I can determine. The twin plastic battery boxes FWC installed in my 2016 Fleet battery compartment offer a (very) little wiggle room; not enough to install anything significantly different than a standard 24 without seriously monkeying-around with the battery compartment.
2) Brenda Cary at FWC very promptly responded to my question about replacement batteries. In brief, she said FWC now offers US Battery AGM 24 for $245/ea that would fit my 2016 Fleet, but they don't ship them, so no help unless you are going to go to the factory. I looked these batteries up on the web: US Battery; AGM Deep Cycle 24. All the specs look good, but good luck finding them! I went through the US Battery web site, got sent to the dealer nearest to me (Tucson AZ at the moment); called them up; very helpful guy explained that that particular battery is gosh-dang hard to find (took him ten minutes), but confirmed that the FWC price of $245 is what they would sell it for if I gave them at least three weeks to track some down. Fugedaboutit.
3) My current plan is to go with the Exide Nautilus 24 (Home Depot $149). Their specs aren't as good as the US Battery AGM 24: the Exide is "dual purpose" whereas the US Battery is "deep cycle" and as Vic pointed out the latter is typically better for camper use. But for $100/each cheaper I hopefully can live with that. Also, I assume if have problems it will be a lot easier dealing with Home Depot than US Battery, an outfit whose web site does not engender a lot of confidence. (Insert the sound of wishful thinking...)
4) How many PhDs does it take to change a battery? Well, it's taken this one a whole lot more time than he expected. If I was independently wealthy and/or liked to spend my time rewiring my truck and my camper, messing with electronic components, and rebuilding my battery compartment, I'd have taken the plunge and gone deeper into studying the vast WTW literature on converting to lithium. I started it, and I have the greatest admiration for you dudes that know the difference between Amps and Volts, but its Greek to me. (I know Sanskrit, Tibetan, Hindi, and some German, but I flunked high school algebra three times.) To anyone who has read this far, hope it is useful. Now for me it's back to staring at the desert, the reason I got my camper. I'll post a follow-up follow-up if the Exide Nautilus does not work out, and this saga continues...