This is a special adapter that connects two 30 amp outlets to a 50 amp outlet. By using adapters, 15 amp outlets can also be used. If two 30 amp outlets are used, you could get up to 60 total amps into you 50 amp service RV. This is not the normal 100 amps capability but better than only 30 amps.
Please NOTE the two two HOT pins will be feeding separate legs on the RV two separae 50AMP Electric ZONES in the trailer.
Like mentioned above you can use an adapter on one of the 30A MALE PLUS to adapt to the 20AMP Service plug.
You can find sources on-line for a molded version cheater cord like shown here...
As long as the rules are followed by keeping both HOT wires separated this idea is safe to use...
What you don't want to ever happen is having one MALE side plugged into a power source and it presents AC power to the unplugged MALE SIDE plug. This would be a dangerous situation. There should not be a way for this happen with a proper setup using the two 50-AMP SEPARATE ZONES in the RV 50AMP Distribution panel.
Usually speaking if you can order one from a well known suppplier like AMAZON it will be safe to use... Ordering one from home made EBAY source might be risky...
Roy Ken