You have a 20a breaker on the main panel for the inverter's output, but what is the circuit in the RV that the new breaker protects, and what is on that circuit? How does that circuit's wire (12AWG yellow supposed to be) also wire to that same new 20a breaker?
Would the breaker when off be able to stop inverter power from getting to that circuit? Not like a Main and a Branch?
Some folks use a sub- panel with circuits on it to run from the inverter. That way they can leave the converter, etc, on the main panel and not have that on the inverter.
The 120v wire from inverter output to the breaker has a ground wire (bare copper) in it. That ground wire should go to the RV frame, same as the 30a Main breaker's wire does. Neutral goes to the N buss, Hot to the breaker.
I have a 2000w PSW and a 300w MSW inverter. Both show two yellows (proper) with the three light tester in their 120v receptacles.