Start with the battery. The furnace control board can't fire the igniter or hold the gas valve open at the furnace if voltage is below around 10V DC.
So, assuming you have shore power, the Power Converter should be changing 120V AC into 12V DC to operate interior lights and the furnace, the thermostat, etc. If the interior 12V DC lights are dark or very dim, you're not getting any power out of the Converter. And since the Power Converter also keeps the battery charged, a Converter failure would be masked until the battery was run down.
Things to check:
- Power Converter's circuit breaker in the main circuit breaker panel inside the coach. Flip off and back on.
- If still no lights, etc., you'll need to check that the Power Converter is plugged in. To do that, you need to take down the rear wall of the pass-through basement storage (on most trailers).
- If it's plugged in, check the outlet to ensure it's getting power. You can do this with a volt meter, or if you don't have one, by plugging in a lamp or small appliance.
- If the outlet has power, check the 3 on-board blade type fuses on the Power Converter. Do a visual inspection. If they look ok, also check ohms with a meter.
- If fuses look ok, check voltage where the wires connect to the Power Converter. If nothing there, your Converter appears to have failed.
Workarounds until a failed Converter can be replaced:
You can recharge the trailer battery by connecting the trailer umbilical and running the truck on high idle for 30-60 minutes. Unplug the umbilical when done. Even better, use jumper cables to go directly from truck to trailer battery.
Get a battery charger and connect it to your trailer battery. If there are several sizes, buy the largest. The battery charger should provide enough power to run the furnace and lights. Run the water heater on electric and don't use any more lights than necessary.