The condition you see is normal but you have to do something about it. Batteries should be on "disconnect" to lessen discharge from phantom loads. The self discharge rate mentioned is more for 12v batteries than 6v batteries.
The batteries should be recharged at least by the time they get to 12.2 volts (60%). It would be better to do it at least every other month during storage to keep sulfation from occurring on batteries below full charge.
Better yet, get a solar-panel of at least 10 watts to keep fully charged (12.6+v) battery(s). A larger panel would be needed to help recharge batteries with some discharge.
When we owned a trailer I normally removed the batteries and stored and checked them at home.