Batteries won't stay charged

In older RVs there is a diode that goes from the fuse block to the alternator, usually it's spliced in the wire just before the alternator. That FIELD voltage is what tells the alternator how much voltage to output to charge the batteries. The voltage source to that device is from the batteries. If it is open circuited (diode bad), or if a connector is bad, or the wire broken, or even a dirty connector at the fuse, that will cause your symptoms.

Test is to put a meter on the starting battery a few minutes after you start the engine. You should be reading 14V or so. Nothing less then 13.2V though.