Below is a diagnostic approach for newer Montanas. Much of it applies to your older model. The two major differences are: you don't have a disconnect switch in the Convenience Center (as you don't have that either); and you converter is likely a stand alone unit located near the converter but not integrated into it.
The 12V flow from the converter to the batteries is pretty straight forward. As others have pointed out, here are the places to look. All connections should have 13.5 +/- Volts if the converter is working.
- The converter is part of the power distribution panel -Ensure the CB for it is On
- There are Reverse Polarity Fuses on the converter which protect against connecting a battery backwards. - Ensure they are not blown.
- Check the output voltage at the converter. - it should be in the 13.5V DC range.
- The power then runs to the Disconnect Switch in the Convenience Center. - Ensure the Red Key is in the position where it can *NOT* be removed. If removed, this interrupts the power charging the battery. The Disconnect Switch is not know to fail, but you can remove it to check there is power in and out of it just to be sure.
- The power next flows to the group of 50 amp self-resetting CBs behind the battery. We all know about the one that hiccups and interrupts the hydraulic system. That one also passes power to/from the converter and batteries. You want to check it and it's brother (it is in the current flow to the battery) to ensure there is 13.5V on *EACH* side of the CB.
- The last connection is the one from the 50 amp CBs to the batteries. Ensure you have clean connections and the cables are not corroded (watch out for internal corrosion under the shrink wrap).
Somewhere along this path there will be the problem. A simple VOM should find it. I hope this helps.