Victorwendel

i had the same inverter. and used lead acid batteries. they where also of age, and didnt last very long. 
 
the way i see it. the mecer units are rather an UPS, than an inverter. their intended use is to keep the equipment running in the event of an power outage. till you can switch off the equipment or till a generator kicks in. yes, if you have an lithium battery connected, the can run for hours. yes f you have enough battery capacity they can also run for hours. BUT. their charging capabilities are not good. if you rush 20a charge into them, they charge to the voltage setting for a very short duration. and then goes to trickle charge. hence, no absorption. i have monitored the charging with a shunt. it reaches the max voltage, and the amps drops off shortly after and the voltage then reduces to a float voltage. and charges with 1/2 amps only. that way it will take forever to reach full charged capacity. much longer than what we have between load shedding. 
 
the very same happens with the Axpert type inverters on lithium battery when charging with a higher amperage. once charging hits the set voltage, it drops to float voltage very quickly and no absorption time. most of the times the charge algorithm is also not triggered if the voltage doesnt drop low enough. thats why i have set float voltage and absorption voltage to 3.45v per cell. that was it will reach 99% SOC, although it charges a little longer. it wont over charge, and wont experience the premature float bug as they call it. 

Try to go lithium, over the lifetime of lithium it can charge/discharge around 6 times more energy than lead-acid/agm/gel, but only about double the initial cost, so 3 times more cost effective.

Thank you for the reply and advise. I put one of the batteries on the Bosch charger last night. Even though according to the inverter the battery was already fully charged it still took the Bosch charger about 5hrs of charging before it went into full/trickle mode. This makes me wonder if the inverter really packs the battery up to full capacity🤔. Im letting the battery rest today before I take a reading tonight, I will then put a 500W load on my inverter and see what happens.
 
 

With the batteries in use for 2yrs you are very fortunate that they lasted that long. They are just at the sell by date. 

I will charge them up fully then take them to Battery Centre to do a load test.

See how it goes after a good charge on that Bosch charger if it has a desulphate setting. It sounds like your batteries have just reached EOL however. SLA/AGM batteries just tend to nosedive in terms of backup time when they get worn.
If your batteries were used together in the same conditions, it makes sense that they would age in unison and both show symptoms at the same time.