If you want to avoid the trip for repair, it sounds like time to go about things in a more organised way. That is made more difficult when we are dealing with a trailer as we don't get the nice drawings that are posted for many motorhomes. That makes it much harder but still something that can be done, just takes longer and more thought.
You've told us several things that hint at how you are working but there are details left out that can be killing your search.
Depending on what I know about the wiring and where things are setup, I would want to start at one end or the other, either where the charging is coming FROM or where it is going TO.
Logically, I might first want to know that the batteries are capable of holding a charge, as they may have gone flat and are now damaged. To do this, I would set a charger directly on the batteries and let them charge for at least a day, then disconnect the charge, wait several hours and verify that the voltage is at least 12.5. As an alternate taking them to have a load test is also a good way to go if parts stores are doing it free and handy for you.
Once the battery is known to be good, The power most often goes to some form of fuse or breaker and that is often closeby as builders don't run wires longer than the have to for economy. Find out if the battery voltage is getting to that point and proceed to step along down the trail until you don't find the voltage and then look for the problem between those two points.
Testing is far better than assuming, so when you have a box open, stick a meter into the joints to verify that the voltage is there. Just because you see a connection, doesn't mean it is a GOOD connection, so if you really want to be thorough, you ay be well off to make sure of each connection on the path you are chasing.
If not able to sort out where the wires go, is it worth the time, to get a tone for wire tracing?
https://www.ebay.com/i/264762535845?...xoCBQcQAvD_BwE
Lots of online videos of how to use the tone.
It's a bummer, for sure, but then the alternate is taking it in for repair!