Do you have a Multimeter? If so start the bus and place the leads to the battery. On the DC Volt setting you should be reading between high 12-14 volts, any less and your alternator will not keep up with the need of the system. If it is low, then you need to start tracing back, the next culprit is generally voltage regulator, belt tension, or wiring.
Also do as

says and check for a drain. If it is a drain then it should be a Major one. A quick way to tell of a large drain is if the battery sparks when you attach the cables to it.