FR 50 to 30 amp warning

And yes, the NEC does cover RVs. You can view it for free on the NFPA website if you want, it's NFPA 70. There is a dedicated section for RVs and for RV parks, Article 551. If you look at 551.71(C) it requires all RV park sites with a 50A receptacle to also have a 30A receptacle. Compliance with that is on the park owner not you. So, you should never need that dog bone in the first place if the electrical system in the park complies with the current version of the NEC.

I'm carefully wording this to say "current version of the NEC" because the requirement for both 50A and 30A circuits on the pedestals only went into effect in the last code cycle, which was 2014. So its clear to everyone, property owners are not required by their authority having jurisdiction (typically the city or county building department) to modify their electrical systems every time a new code version comes out. Only new work is covered, and often the code version in effect in a jurisdiction is one code cycle out of date. Their insurance companies might have a more stringent requirement though. And if they start losing customers that would be a real incentive, so rather than use a dog bone, you may want is to politely tell them that they are not in compliance with the current version of the NEC and go stay somewhere else.  

Also 551.40(B) requires all electrical system components, including any cords and adapters, to be listed for the intended use. Compliance with that is on you, not the park owner, since you are providing them and connecting them. 

I've never looked at a 50A male to 30A female dogbone (and I don't plan to buy one), is that thing actually listed? If so, there should be a mark on it, typically in the US it would be UL or ETL. And there should also be instructions and use limitations that came with it, those are required as part of the listing.  What do those say about how it is intended to be used? I'm curious because right now I don't see an application for it that assures safe use.