You may well be discharging your battery more deeply than you think, hence the short life. 11.2v open circuit for a SLA is well below fully discharged at around 11.7 to 11.8V open circuit, but your meter is probably showing you the voltage with a load on so you really don't know how deeply discharged the battery is. If you want to check the actual approximate state of charge, put a multimeter on the battery after it is disconnected from all loads. The table in the following article shows an approximation of state of charge based on open circuit voltage (from 100% @ 12.7+V to 0% at 11.8V) for various types of LA batteries.

Doh. Forgot the link. https://www.energymatters.com.au/com...age-discharge/