Flooded lead acid batteries in series and parallel - voltage drop at negative terminal
You have unbalanced batteries. The lowest capacity batteries are running down first. Your batteries may be good, but unbalanced due to charging history. Or one or more of your batteries, or their individual cells may be bad.

Normally, lead acid batteries are balanced by controlled over-charging. Flooded cells gas freely when being over-charged. This is expected, and safe, as long as the gas doesn't collect in any volume (it's explosive), and as long as you top up the cells to the correct level periodically with distilled water (water is lost during overcharging).

25.8v is not enough to overcharge two 12v batteries connected in series. Over time, small normal differences in leakage current will unbalance the batteries if an overcharge never happens.

First, check all your batteries are topped up to the correct level. Fully charge each battery until they have been gassing freely for some time. Now check the specific gravity (S.G.) of the electrolyte. Compare all the readings with each other, and with the expected readings from the battery data sheet. This is where you may learn whether any individual cells are bad.

If all the S.G.s are reasonable, now that you have fully charged the batteries, reconnect them and see what happens again. Ideally, your end of life trip should measure the voltage of each battery, and stop when the first goes below 11v (or 10.5v if you like to live dangerously). Tripping on the total voltage of a series string is not safe.