Update - thank you all for the help.
Had the batteries tested at Batteries Plus Bulbs - both showed a bad cell. Batteries less than 2 years old. But no warranty, no pro rate on golf cart batteries.
Went to Costco and bought 2 more. I was a fan of the Duracell before today. Even though they have more AH, 18 month old batteries don't get me buying new ones. Saved $60 buying at Costco.
But, I still need to figure out why this happened. Did my trickle charger fail me this winter? Although I tested with a meter and 6.3V seemed OK. Is the WFCO failing me? The batteries have only been in the trailer since the end of March, so only a few weeks this year. Could a converter kill the batteries that quickly? Can a bad cell be caused by the converter or charger? Not sure but will research this.
From what I am seeing, the upgrade to a PD4635 converter would be a worthwhile investment? It's not that bad and there is a direct replacement if I can find one.