I've seen lots of starting batteries give Ah ratings, and assume they are at the 20Hr rate.
I'd not intentioally cycle any flooded Starting battery, and the presence of many 12v ports is hardly an indication the manufacturer decided a dual purpose battery is warranted. The new trucks which now come with 1000 watt inverters do not even use a dual purpose battery afaik.
My well aged and used Northstar AGM is definitely on the slide. Mostly this is obvious by the voltage it can maintain when starting the overnight cold motor at ~180 amps of load. It has no issues stating the engine but now at 40Ah from full drops to 8.7v before engine catches. If fully charged this is 11.7v.
I have an 18Ah AGM, which can, by itself, just barely start my overnight cold engine in warm ambient temps. If I have this in parallel with engine battery starting overnight cold with both batteries fully charged voltage stays above 12.2v cranking and it cranks much quicker.
As far as voltage held overnight deep cycling, I'd not know this battery has ~1100 deep cycles on it. Still rebounds well over 12.2v at 45Ah from full once the major loads are removed. This is as long as it sees regular full charges and regular high amp recharges from a well depleted state. If it gets lots of low and slow solar only to full the voltage held under load for Ah removed, is not nearly as impressive. Rectifying 'not impressive' means 40+ amps applied to a battery with 35 or more Ah removed from it, until 14.7v is reached and 14.7 held until amps stop tapering. more aps applied from a more depleted level for longer is more effective in rectifying 'not impressive', at least on this TPPL AGM battery.
At its advanced age I cannot get it to taper to 0.4a at 14.7v anymore. It settles at 0.8a or so and then starts rising, though the high amp recharge will lower this plateau for a few cycles before it starts climbing again.
lowering voltage to 13.8 from 14.7 once amps started rising, 12 hours later will have the minimum taper amperage at 14.7v slightly lower than before I lowered voltage.
