Hi jas8796,
If your interior lights and other 12V systems work when plugged into shore power, but don't work when shore power is disconnected, the 12V mini-circuit breaker that sits in between your battery and the fuse box has tripped. It's usually a manual reset breaker with a teeny-tiny reset button. Another possibility is that the battery cutoff switch is OFF.
Here's a picture of the manual reset breaker that's on the buss bar near your battery.
Also attaching a picture of the buss bar and mini-circuit breakers. Sometimes they're covered with a rubber boot. Yours could look different; the configuration varies by trailer model and floorplan.
Our owner-written
12V Block Diagram and Diagnostic Guide explains this problem in detail and also provides a diagnostic flowchart in case there's something else wrong.