Gotcha!
You are already fuming but the new breaker on the adapter will not protect against hot neutral reversal plus a neutral to ground fault within the rig! You are 100% correct.
But a ganged dual 240 breaker rated 30-amps with one pole used for neutral WILL work. I waited to post this separately. Just as a means to get a person to think.
HOKAY. What are the chances of having a more than 30-amperes potential neutral to ground fault? Hot entering via neutral and then the potential of having the continuation of the circuit through BOTH the power post miswired in reverse L1 PLUS ground?
I'm not worried about anything downstream of the splitter. It' s the splitter to power post overload I am concerned about. Camping. With BOTH legs protected by a ganged breaker the chances of a fire are reduced to near ZERO because of OVERLOAD. No matter the cause of the fault. Put a ganged 30-amp breaker in series with the hot and neutral before the splitter and eliminate the problem.
I suffered a neighbor's fire in an RV park down here because of this exact fault. Splitter caught fire then the coiled power cord insulation caught fire and blackened the side of his fifth wheel. Oh lucky me. I was parked on the fire side of his rig. The park had 30-amp MISWIRED receptacles and connected grounds.
I decided to bring this entire issue up on this thread. And before you post about "what about houses" take a moment and think about a MAJOR meaning a PRIMARY hot neutral reversal LIKELYHOOD.
There! It's off my chest now...
A tip. A permanent mounted neutral to ground panel mounted voltmeter has never ever been mentioned in this forum. It is to reveal faults. Hmmmm...