The 50A circuit CAN provide 250V, and there are a VERY FEW big 50A rigs that use it.
But that is NOT THE NORM! For nearly all 50A coaches, that system is TWO 125V/50A circuits. Your coach uses ONE 125V/30A circuit. That "A-Dapter" (I can't help quoting Johnny Cash) uses ONE of those 50A/125V circuits for you to plug your 125/30 into. The other 125/50 "Leg" is unused.
And YES it is safe to do this. 50A will not over-pump your electrical system since your coach's on board power panel's main breaker is 30A.
What's NOT safe is applying 250V to a 125V system, and the posts above have covered that.