Its funny how we tend to pick a statement apart, word by word in order to find a fly in the ointment.
Yes, technically the 50/30 adapter is not a step down in voltage but a step down in that a 30 amp RV can plug into a 50 amp shorepower.
Yes, you indeed do get a full 50 amps of power to your RV and this is technically not legal because of the size of wiring that the dogbone is connected to going to your RV. Yet, that 50 amp once it reaches your RV is completely protected by your 30 amp RVs electrical setup. So if there is any overloading possibilities, its only going to be in the power cord to from the RV to the dogbone adapter.
Since the 30 amp RV is only going to use potentially 30 amps, even that power cord will never see a draw of over 30 amps so its pretty much a wash.
Bottom line? Use the dogbone if you like, or plug directly into the 30 amp receptacle at the post. And don't get confused by the nit-pickin' over something as trivial as this legitimate question.