That is much faster than self-discharge should be.
Are you sure the batteries
are 100% isolated, no monitoring gear
attached? An actual fully-isolating bank switch right near the bank is best.
Have you been seeing other evidence they are nearing EoL?
Were they actually at 100% Full SoC 6 weeks ago? How do you usually verify that?
Was the temperature much warmer then?
Can you get some solar