Hi Budman,
There are some 12V DC mini-circuit breakers near your battery, that are covered with a red rubber boot. One of them has a teeny-tiny reset button. When that breaker trips, the battery doesn't get recharged, but all of your interior 12V devices and lights work while on shore power. I've attached a picture.
It's also possible there's no output from the Power Converter.
Inside the trailer, you've got a test panel that shows the holding tank levels. On the right side there are 4 lights that show your DC power state. If on shore power and the Power Converter is working, you'll see 4 lights when the test button is pressed.
If the battery is depleted, and you see only 1 or 2 lights on the test panel, the Power Converter is not working. Check the main circuit breaker panel inside to ensure no breakers there have tripped. Another possibility is that the battery was momentarily hooked up in reverse polarity. That will blow the on-board fuses that protect the Power Converter.
If none of this helps, we have an owner-written
12V Block Diagram and Diagnostic guide that will step you through how to figure out what's wrong.