The first place to start is welcoming you to irv2, and hoping once this issue is resolved, you enjoy the RV lifestyle for may years to come.
Now, to your problem. You did right by checking the fuses first.
Solo2 mentioned a grounding issue. This has been a problem with several of the E class FORD platforms.
I don't know how handy you are with a 12 volt circuit tester, but it is an essential tool when dealing with 12v electrical lighting and trailer wiring issues.
Let us know how things are progressing.