If you use a 30 to 50 amp dogbone there is a small chance (think one in the entire population of the world, 2 at most) chance that you could get a partial short in your 30 amp cord that would cause damage and not trip the 50 amp breaker.
This is offset and further reduced by several facts, one of which is that the cord is rated for 30 amps in a WALL, and it's in free air. which gives it greater heat dissipation ability. The 30 amp breaker inside your RV will protect you.
HOWEVER: Owner's manuals are proof read by LAWYERS who's motto is "Cover your .... Rear exposure" and thus. that is why they give useless warnings.