Well lostagain, no math involved here really so not to worry.
The drawback with my suggestion is cost and weight, long 50A RV cords are not cheap or light. Your suggestion uses only a short inexpensive input cord.
Best would still probably be a NEMA 4X or IP55/IP65 style enclosure with a breaker in it, your 50A pigtail and male plug, and another short 30A pigtail with 30A female out. Basically you're creating a dog bone with a breaker in it, with the breaker in a nice solid enclosure that you wouldn't have to worry too much about getting rain in or mechanically damaging.
There are some reasonably priced ones on Amazon. Like these. You'd need to pick one that fit the breaker you've selected and have enough room for terminating and routing the conductors.
https://www.amazon.com/Ogrmar-Plastic-Dustproof-Junction-Enclosure/dp/B07871XT6F/ref=sr_1_2?s=industrial&ie=UTF8&qid=1549455539&sr=1-2&keywords=ip65%2Benclosure&th=1