Thanks to those that answered my question. Sounds like even though the total watts available on the 20amp 220v outlet is higher, the option to the RV to use up to 30 amps selectively would be better. I didn't know how the 30amp to 50amp adapter was wired that makes sense.
As far as why I don't just put in a 50 amp service, access to my place is questionable to all but the smallest of 50amp RVs so probably would never be used. Accept for the 30amp RV box I already had everything else I needed to put in the the 30amp service.