How do you use the 30 amp cord as an extension cord with a "reducer"?
The "reducer" is called a dogbone adapter but is usually made to convert the male end of your 50 amp cord to a 30 amp female RV receptacle. I suspect what you want has a 50 amp twist-lock to plug into the coach with a 30 Amp plug to plug into the 30 amp Extension Cord? I haven't seen a dogbone like that but anything can be made.
Actually, it seems like Amazon has everything now:
https://www.amazon.com/Power-Adapter.../dp/B00QEH0QB6