You either have more demand than you are producing or the controller needs setting higher.
This type of on off controller tends to let things get undercharged. 28.4v is just about enough to charge but it needs maintaining at this voltage for a fair time. If you are tripping off at this voltage then you will eventually become discharged. If you equalise regularly it may be just enough. The 26.5 is reasonable to come back on if you have load removing the surface charge but if there is no load then you are wasting a lot of your potential charging current.
As you have been able to measure the sg you must have flooded cells and you will do them less harm if you raise the voltage settings. I don't think it will hurt to raise the top point to 30v as far as the batteries are concerned, just keep an eye on the sg and make sure it comes up and stays up, if you use too much water and the sg stays up you can reduce volts a bit.
The immediate snag that I can see is that your inverter may trip on over volts so you may have to compromise.
If you can reduce the differential between the switch on and off points then you may be able to switch off at 29v and on again at 28.
If you have trouble with the inverter tripping then you may have to set your controller higher at times when you are not using the inverter to get the batteries back up, they need a good equalising at present on the first windy day, the first thing is to get them back up and then try raising the settings to keep them up. If that fails then you must be using more than you are producing. Output from wind generators is very variable so it is often difficult to judge exactly what you are producing.
If your batteries were not used much and spent most of the time floating your settings would probably be fine but you are using significant power and you may be compromising much of the later point of the charge cycle. With all controllers you need a bit of manual intervention so keep an eye on the sg. Simple controllers need a bit more watching than the clever ones.
Flux