Need more information about your batteries. Are they AGM or regular flooded cell batteries, Have you checked the water level in them lately? I'm still using the batteries that came with the RV in 2013. I've almost replaced them 2 years ago, but after I performed the annual maintenance that I had been neglecting on them they seem to be holding on. Now I do have to add that I have a 165watt solar panel maintaining a charge on the batteries also.
Now back to what happen, was that I didn't check the water level in them and they stopped holding a charge, also around the same time my ATS and shore power cable fried due to loose wires. There were a few other electrical gremlins around that time also. All do to me not doing annual maintenance checks on the electrical system.
So what you need to do is see what type of batteries they are and if flooded cell batteries check the water level, if low add distilled water to them and then get the generator running an charge them up. Then let them sit a few days and repeat. You might be able to rejuvenate them back to life or you may have to spring for some new ones.
If you are not going to do the required annual maintenance then spend extra and get AGM or go even a step better and get LiFePo batteries.