Some good some well meaning some misdirection in the above replies
Question: Can you plug a 30 amp rig into a 50 amp outlet using the proper adapter: YES you can many do.
Question: Wont the voltage difference matter:
Answer: No, the adapter takes care of that See technical notes below
Question: Is it not dangerous
Answer: no, you have a 30 amp breaker inside your rv, There is one failure that might cause an overload on the cord but it is theroitical. I have never heard of it actually happening. (Partial short in the cord or before the main breaker in the RV.. In practice that's only possible on a test bench, a real short will trip the 50 amp breaker before the cord is damaged)
Question: Why would someone wish to do this
Answer1: I have been in parks where the 30 amp outlet was kind of nasty, the 50's are usually better condition.
Answer2: The plug/socket connection is better.
Answer 3: Though I've never seen one myself. It's possible that a site might only offer 50 amp, no 30 or 20.
All 3 levels, 20, 30 or 50 have a safety ground wire (Green or bare) This wire , in theory, carries NO current (Or rather it should not, if it does there's a problem) So we will ignore it, On your plug it is the "Rounded" pin.
50 amp service has 4 wires, the ground described above and 3 flat pins
The voltage between L-1 and L-2 in a properly wired park is 240 volts.. The voltage between either L-1 or L-2 and NEUTRAL is half that or 120 volts.. Just what you need.
The dog bone adapter picks off just one leg (L-1 or L-2 L stands for Leg or Line we usually say Leg) and the neutral it ignores the other one completly.. Save for one dual 30 amp to fifty adapter (50 amp plug 2 30 amp outlets) that puts one leg on each 30 amp.
30 amp servce has 3 wires, The ground
One of the legs
SO it' is 120 volts
In a triple box (20-30-50 amp) the same wires that feed the 50, feed the 30. and 20,, Just one leg feeds the 30 and depending on who made the box it either also feeds the 20, or the other leg does.
Home is where I park it!