Age and condition of batteries is going to be a factor. Newer batteries that have always been well maintained should handle a month or even more disconnected. Unfortunately testing with a volt meter doesn't give you the best indication, a hydrometer test of the cells is the best indicator of battery condition.
As someone else mentioned, use of a trickle charger, if power is available, certainly helps. You also need to be aware that the disconnect switches don't disconnect everything, and sometimes there is still considerable parasitic draw on the batteries. This is most often caused by aftermarket installations that connect power on the wrong side of the disconnect.
When you have the opportunity where power is available to charge the batteries, use the disconnect switches and after a week check the battery level to see how they are doing. If they have dropped too low you at least have power available to recharge.