As an electrician I'm very comfortable wiring up generator connections to the supply plug of our Montana 2955RL, and you might look at the post here as to how a 30 amp generator feed can supply a 50 amp input plug.
https://www.montanaowners.com/forums...ad.php?t=86634
All #12 wire is not created equal. Depends on insulation and wire design. I have used #12wire rated for 30 amps in free air but the pics you supplied appears to me that the wire may be undersized as you said.
After looking at your pics again, I realized that none of the wire nuts were very secure. In an RV traveling down the road vibrations can loosen wire nuts and create problems with shorts, grounds and open circuits. It is always best if using wire nuts to wrap them with electrical tape overlapping onto the wire being joined together at the wire nut.