6v battery fail

My last set of 6v batteries lasted me 6 years and were still working good when I sold the camper. (they came from the dealer when I bought the camper) Year and a half ago I bought a new 28 foot TT and it came with 2 6v 220 batteries. The first year they worked fine. But they have failed this year. I stayed on shore power for 6 weeks but before I left I researched the converter docs for the trailer and they say they will maintain the batteries once fully charged and will not over charge them. The batteries were never dry. I didn't notice they were weak until we started heading home.(the furnace fan seemed to be running slow). The temp dropped to -15C/5F on route so I thought maybe the cold was affecting them and they would be fine once I got them indoors. On the trailer monitor they show full charged with the truck plugged in and running in high idle. But with the truck unplugged they show "fair". When I got home I brought them inside and plugged them into a CTEK charger/keeper, (the same one I used to store the other set of batteries for 6 winters.) And after 24hrs they still showed fair on the specific gravity tester. Then I put on an old style 2/10 amp charger at 2 amps and after 12hrs they still show the same. All the cells read exactly the same. They all show weak. There are no dead cells. I called the RV dealer and of course he says my 1 year warranty is up. (But he is willing to sell me another set.) Any ideas or thoughts as to what happened to these batteries. I don't think they where overcharged as the fluid levels where up for the whole trip. I am worried if I invest in another set the same thing may happen again.