Sounds like the battery voltage is not getting to the trailer/converter.
The 12V inside the camper will be sourced by either the converter (from the 120V, which is working), or the battery (not working).
There are usually a few connections comming off the positive lead of the battery. One of these goes to supply most of the trailer (lights, fans, etc), and some are dedicated for larger or external loads like slides, power jacks, stabs, etc.
Based on your symptoms, I would say that the inline fuse/breaker (located on the power lead off the battery) is blown, or the battery connection is loose/disconneted for the main trailer lead. Go to the battery check connections, and check the inline fuses, usually located near the battery. If it looks like a rectangular block with screw terminals, it may be a resettable breaker. Reset it, and check 12V operation with AC unplugged from the trailer.