12 volt system

Don’t know if this will help you or not, we had a similar problem on an older trailer we had. If you could get hold of a wiring diagram that would help but as it is quite old that may not be possible.
Trace the hot line from the battery and see if you come across a breaker (automatic fuse) if you find one, power should be on both sides of the breaker. This can be tested with a 12 volt test light. If you do not have one, buy one at any auto store – they are fairly cheap ($10 or $20). Its a good investment for all sorts of problems on your motorhome or car etc.
If you can’t find a breaker or fuse, keep going back your wiring and see where your power cuts out. it is also very important that you have a good ground – so do the same with this cable and see where it is ground to and clean both surfaces. Use emery cloth or even normal sandpaper or scape the surfaces clean. Good luck. Ron R.

The battery in my Ford has a large knife switch to isolate it from the house. I have a starting battery, on the passenger side under the hood, with that switch and a house battery on the drivers side under the hood. If the house battery is not connected to the fuse panel you wouldn’t have 12VDC unless you had shore power running the converter which is with my fuse panel. When you are running the gen or plugged in the converter is producing the 12 for the lights.

I have an 85 Coachman Class C. My lights are all 12VDC. I would try to check the fuses in your electrical center.

Locate the converter/charger in your RV. It should be a brown or black panel with both fuses and breakers under a door that will open. It usually has a small twist lock to access the fuse and breaker panel. Check that all your fuses are good for the 12 volt side of the panel.
Also there may be an auto switch built into the converter or a separate unit that can also stick in one position or the other – which can sometimes stop either side of the system from working properly. If that is the case you can sometimes plug in your RV shore power cord and unplug it a couple times to get the switch to start working again. I had this problem on an Itasca class C that I owned for 14 years.
If none of these issues solve your problem – It could be that the converter has an internal short somewhere. Worth a check in either case.
Electrical issues on either system 12 volt or 120V can be a bear to track down sometimes.
Another thing to check is whether your 12 volt charger that is built into the converter if so equipped is functioning properly. Easiest way to check this is if your battery current rises when plugged into the shore power or while the generator is running. Hope this helps track down your problem.