50amp to 30 amp dog bone

If you bought a separate breaker and mounted it in a NEMA 4X watertight enclosure and used water tight cable glands to bring the cables out that would be better. The breaker you picked wouldn't work though, its designed to snap onto the bus bar in a home panelboard. You'd need to find a back mount breaker and attach it to a back plate in the 4X box. 

If you're worried about having nuisance trips of the 20A inline breaker, I can say that I've run my trailer on a 20A circuit at home a lot and have never had the breaker trip.  Although I haven't tried to run the microwave and the a/c simultaneously, its not a big deal to turn off the a/c for a few minutes when running the micro, I have to do that when running on my Honda 2K genny.

OTOH, don't be afraid to change out your connectors,  it really isn't hard. If you've ever reterminated a cord, its the same kinda thing. 

When I said leave one of the hots disconnected on the 50A 240V connector, don't let that confuse you.  The two hots are the pins opposite each other side to side, the neutral and ground pins are opposite each other top and bottom.The ground pin is the round one. You get 240Vac between the two hots and 120Vac from either hot to neutral.  Since you only want 120Vac out, one hot is unused. 

When you cut off the cable on the Rigid part, you will see a green, a white, and a colored wire (usually black but sometimes red).  So, you connect green to the ground pin, white to neutral pin, and the colored wire to one of the two hot pins. Doesn't matter which. The other hot pin will be unused. That will give you 120Vac out same as the dogbone does.

On the output connector looking at the end face of the female receptacle with the round hole up, the neutral is the pin on the right and the hot is the pin on the left. So, white to neutral, black (or red) to hot, green to ground, same as before.