Fascinating. Can you tell us more about your batteries, type, configuration, how it is used and so on?
Small 12V system with 1500W solar, I have 5 FLA and 4 AGM batteries in parallel. This is primarily used to run a pool pump but also to run a 10K BTU air conditioner in my shop occasionally. Yes the mixing of FLA and AGM batteries is another thing some people say not to do, but yet It has been working fine for me. I can test the SG of the FLAs easily enough, but the AGMs I just put a load tester on once every 6 months so. As long as everything keeps working (and it has), I don't worry about it. My current FLAs are between 2 and 6 years old. When the oldest one gives it up, I will just replace it.
IMO proper maintenance is the key, along with not over-discharging.
<edit> Stupid auto correct