Help with battery Ah de-rating after many years.

Hi all, I'm finally getting around to installing a battery monitor (Victron BMV-700) after far too long. I'll need to input the Ah capacity of the bank. I understand this is usually done base on manufacturers spec's @ the 20 hr. rating. I'm using 6 x 6v Trojan FLA, L16's configured to 12v with a nominal capacity of about 1100 Ah. Here's my dilemma: these batteries were installed in 2012 so they've got more than a decade on them. I know... you're going to say they're finished, replace them. However, I've babied these, I just don't know how well (since I didn't have a battery monitor on them - bad me). Based on the daily charge, about 1.2kwh, 80ish Ah, they are seldom drawn below 90%. A few times a year, they get down to 12.5v for a day. We have an off grid camp (cabin), used regularly in summer, not in winter. Ambient temperatures seldom exceed 27c/80f. Winters are "cool", I'm guessing the batteries stay at about -15c/5f. The batteries ask for about 4 oz. of water spring and fall. All this to say, they have a pretty good life... they show few/no signs of degradation (but obviously they have based on age).

Back to the battery monitor, I need to input the Ah rating of the battery bank based on a degradated number (I assume), can anyone take a stab at that based on experience? Or is there a method to actually calculate the "remaining" capactiy without drawing them down to zero under load?

I know this is a "how much does a car cost" question, but I thought I ask.

I'll throw one more in here, should I expect the batteries to fail quickly or slowly degrade (becasue of their age)?

Thanks for your time!

Wow, that's definitely "How Long Is A Piece Of String", but I'd assign them half (550AH) and use them till they fail, they really don't owe you anything after 12 years, that's a pretty good lifespan!

I mean, you could give them a good equalizing charge, let them sit 24 hours, measure the SG with a refractometer like https://www.amazon.com/Refractometer-Antifreeze-Ethylene-Propylene-1-100-1-400kg/dp/B01N4RMOA4 then draw them down by 100AH or so and test them again. That might tell you if there are any weak cells. Repeat yearly and you might have advance indication of failure.

Thanks for the input: I see that my skills don't translate to understanding where to post on a forum. Don't think I can move it now.
The odd part about these batteries is they seem to be performing like the day they were installed. I don't see anything that has (noticeably) diminished, performance seems consistent????

If you've only used them 5-10 percent and not very often, and kept them topped off with distilled water, you might not have noticed anything because there's nothing to notice. Say they only have 15% capacity remaining, how would you know? And why would you care?

I had that same thought... how would I know? I know it isn't correct and accurate but generally speaking, my LED's read a low of 12.6ish down from the 13.4 float. And, why would I care? I'd rather not have them fail during the winter and have everything freeze and make a mess. Is the answer to replace them now, perhaps, but I don't want to toss anything that has reliable service life left in it. And before it comes up, yes - the bank is probably oversized but only for summer. In late October it's not uncommon to get 3,4 or 5 days without sun while temps are at or below freezing. This is the only time of year that I "tax" the batteries, occasionally down to 12.5v but the Iota/genny take care of that pretty quickly.