Battery and Power Problems

There has to be an interruption between the 12V distribution panel and the battery or the battery is very low.
An electric tongue jack is usually connected directly to the battery terminals and has it's own separate fuse.
I would suggest to look at the battery connections and under the trailer's frame to see if there is a junction box. The box may contain a circuit breaker or you may discover a loose connection inside that box.

All is well with your 12V devices when plugged into a 120V shore power connection because the onboard converter is powering the distribution panel.

To illustrate all of this to yourself, you can connect a separate wire from the battery's positive terminal and connect it to the main 12V power lug on the distribution panel. You will regain 12V power when unplugged by doing this. That is, as long as the battery is charged.

Reading between the lines, it may be that you had a battery issue, the battery was replaced, and within the course of doing this, the battery -> distribution circuit breaker was tripped. It may have reset but the converter would not have been able to charge the battery while the breaker was tripped.

From your description of the issue it sure sounds like your battery is bad. When you're plugged in the converter is providing the 12VDC power. Sounds like your battery is not connected correctly, there is a blown fuse between the battery and converter, or even a blown reverse polarity fuse inside the converter.

yes that fuse is good, as the front power hitch works whether plugged into power or not. again, if plugged into power all is well.

Have you checked to see if a circuit breaker tripped or fuse is out between the battery and the distribution panel? There is usually one within a few feet of the battery.