I too was faced with the possibility of replacement of batteries. Unlike the OP, at least mine were identical GP24 12V FLA "deep cycle/marine" batteries. As pointed out earlier, that means they are actually "hybrid" batteries, not true deep cycle. I thought the batteries needed replacement. Our iN.Command system monitors voltage and after the batteries reach 11.8V for two minutes, the generator gets an auto-start command. This was happening all too often. According to Keystone, the measured voltage does not take "load" into account. Nor does it provide "at rest" voltage which requires an hour or two of not being used.
Subsequently, I found that there was a significant difference between the voltage reported by iN.Command and as measured with a multimeter at the battery. So, voltage drop in the measurement system coupled with a lack of true "at rest" voltage meant that the system really was not operating on factual information. Even a measurement of voltage right at the batteries was not indicative of at rest potential. A load test indicated the batteries were ok but You can't stop and do this a bunch while camping. Our 180W solar system helped in the day, but forget about running the heater all night!
What I wanted was an accurate measurement system of the true condition of my batteries. I bit the bullet and spent $200 for the Victron BMV-712 Smart system. This system measures actual current-over-time in amp/hours, measures voltage right at the battery, measures both incoming (charging) and outgoing (discharging) current, and provides an accurate accounting of all amp/hours for the system. This is one of the best things I have done with my trailer! Yes, I spend $200 for the monitor - but I saved $400 from not buying new AGM batteries I had intended to purchase. Now, I can check my battery status in amp hours and do it right from my phone. It turned out my batteries were just fine. The combination of excessive voltage drops in the iN.Command measurement system coupled with any existing load meant that the voltage assessment just was not accurate.
So I highly recommend either the Victron or some other quality system that uses a shunt and measures both incoming and outgoing amps in your battery system. User Logan X has a video on this process at: