Recommendations on Kohler generator

Have issue, appreciate some advice.
Generator has been working well. Had an easy winter but still ran a few minutes with a small load a couple of times a month.

starting my spring maintenance and decided to have a reputable co.do a fuel polish on 260 gal in two tanks. All went well. Changed racors, primary and secondary fuel filters on engines, and the kohler fuel filter.

engines started up great. No leaks. Generator turned over a couple of times slowly then starter just clicked. Charger is working, assumed bad battery. Replaced battery today, battery shows full charge but same sound like dead battery.

probably above my pay grade but what should pro be checking first.

3JH2L-TE-K

I believe this is the model number. 18 kw generator

I cleaned the lugs on the battery as good as I could but they still were not “shiny”. Normally I would have taken my small dremel to it it cleaner really well but it was at home. I did not check on the starter. Thanks for the tip!