OK, we're cooking now. According to the WFCO distribution panel you have, it should be used with the WFCO 9800 series converters. They all basically look and operate the same, just have different DC amp ratings. I chose the 65 amp one for the example.
The reverse polarity fuses are on the front/face of the converter....as you can see in the link above.
Just have to find the converter(s) placement now. They can also be in your pass thru storage compartment, or behind a panel accessed from the pass thru storage.
Using the junction box (pic #3 that you posted) as a starting point, see if you can back trace the wiring as to where it goes up into your RV....and see if it's near the pass thru storage.
Of course, if you have an enclosed underbelly, this may be harder to attempt/see.
Here's an example of one behind a panel in the pass thru: