Before buying new batteries, I'd suggest disconnecting and fully charging them (minding Ted's warning). Let them set for a day and then check the voltage (fully charged and resting for a day, voltage should be about 12.7 if in 'good' condition), or better, the electrolyte with a hydrometer. Another test, after charging, is to take them to one of the chain auto part stores and have them perform a load test. All of these will provide some reasonable data. If they checkout, I'd bet you have a load/draw (yes, even with the bus 'off') that is discharging them. Of course, if they are very old (like 5 years), they probably just need replaced.