House Batteries 6volt deep cycle

Thanks all. I was surprized that they are shot. 4 Interstate batteries 3 years old. I keep in storage for 6 months. Plug in the coach (store enclosed at CG), wait 10 minutes, starts right up. Plug into shoreline at CG, come morning, chassis fully charged, but house batteries dead. 12 volt house lights flicker, EMS board flickers some when heater comes on and off etc. All leads to bad house batteries not charging....