Are all the circuits (20A, 30A, 50A) serviced by the same line in a typical RV power pedestal? In other words, one line comes into the pedestal, and it's split out into the 20A, 30A, and 50A plugs. Or, is each one a separate circuit from the central power distribution?
If they share a common feed, you could just plug in a lamp to the 20A outlet and not have to figure out any additional adapters.
Though, a 30A plug to 20A socket adapter would be handy to have to quickly plug something in at the pedestal when the power goes out next time.