If you skin produces dozens of thousands of IU while staying 20 minutes in the sun, and the RDA is not even a tenth of that, I'd rather wage the RDA is completely wrong and misplaced.
Type 1 skin is 2000 IU's per hour if completely exposed to the sun though someone may call the cops. Type 5 skin is 500 IU's per hour but all of this is moot if one does not have sufficient precursor elements for the D3 process to occur. This involves vitamin D2, C, collagen and a few other elements. I don't remember the whole chain off the top of my head. I'm lazy and just take pills.