Two points will help as starters on the chase. One is the year make and model of Rv you have. That will help if we get into it that far. But it is also true that we can look at parts catalog and drawings for some info but then we no longer get the good wiring drawings for that older model. That combo leaves us a guessing game at times.
If you can find a label/tag on the converter to tell what brand and model of converter you have, it is possible we might dig up a drawing or info to show where to find different points to test.
More info can cut the amount of guesses we have to try before getting the right answers!
But some basics that may help to sort what you see, may be good, too!
Idea is that you will have 110AC power going into the converter from the power cord. I think I can kind of guess at the general layout but check it before trusting! That 110AC will feed several breakers for supply going to things that use AC power. Do you see it maybe split into two groups of fuses or breakers? One side AC , the other 12VDC?
Possibly spot the AC wire coming in and then look for how it gets to the converter. Is that converter built into the back of the load center with fuses/breakers, etc.?
If you can find a wiring diagram, that would help us say where to check but we may have to do some guessing.
If you see the power from the cord, does it get to the big transformer I see in one picture? I think that transformer will have higher voltage like near 110AC going in and then much lower like 12-15 Volts coming out.
You mention thinking it is not working?? Before going too deep, I recommend checking for sure the converter is or is not working! checking may point out some point that saves a lot of trouble.
When you check for DC voltage on the fuses that feed things like lights, fans and such, you should see that 12Volts when the battery is connected, whether the cord is plugged in or not.
You might have something like a battery cutoff switch open when not getting power from the converter to the batteries!
Check the battery voltage and then check if it changes when you lug the cord in. There should be jump in voltage if the converter is getting power to the batteries!
You may have already done this but I thought to mention how easy it is to miss some of the small points like having the right switches on!
