Sorry to disappoint some of you, but I've been working with lots of electrical circuits, and know a Single Phase from a Double Phase.
A 30Amp Service is a Single Phase, 30Amp, 120Volt Circuit.
A 50Amp Service is a Double Phase, 50Amp, 120Volt Circuit.
In your brick and stick homes, (as opposed to our RVs,) there would likely be a double phase circuit for an electric stove. What the two phases allow is to let the stove's heating elements effectively attain a 240Volt, (two-phase,) circuit, by crossing the 120Volt leads across each other, netting 240Volts. Older homes may not even be wired with 240Volt service, so it's not the case in those homes.
In our RVs, it supplies our power panel, (via the heavy 50Amp power cable,) with two distinct and differently phased 120Volt Circuits. And since each one is 50Amps, the larger air conditioners will run, and you might even be able to nuke a hot dog while remaining cool.
From what I understand while touring Keystone’s Montana factory, the power panel is arbitrarily split between the two phases, pretty much splitting the load so that one phase won’t be using a lot of Amps, while the other phase is almost unused.
When I had to install a plug for the Fiver, after we purchased it, I only used a 30Amp, Single Phase receptacle, because I didn't want to have to buy a 50Amp extension cord for the RV when we weren't going to be using the AC in Fairbanks, anyway. I used a 30Amp-to-20Amp adapter, so I could run a standard three-prong extension cord to the trailer’s power cable, where I had another adapter to convert the standard three-prong receptacle to the large four-prong 50Amp RV cable. I could have installed the 50Amp, Two Phase receptacle in my garage, and the adapter pigtail that would run it down to a standard 20Amp extension cord that I was using to run to the trailer's and lights, etc, before we left town. But I would have had to have even more adapters.
Adapters, or pigtails, allow you to get power from a low rated outlet to the higher rated cable on the RV, or from a high rated outlet to a lower rated cable on your RV. It does the necessary internal cross-connections that make whatever service your RV requires, the cable will "see" the proper voltage on the proper wires. (30Amp cables having two power leads, and a ground. 50Amp cables with three power leads, and a ground.)
I have a great page for people to see what the RV wiring is all about.
http://www.myrv.us/Pgs/RV/50amp.htm Trust me, it will open some eyes. It explains in pictures what I'm stumbling over in words.
Enjoy the page, and share it with anyone in need of wiring anything for an RV.
Steve