There must always be 240V between the two ungrounded terminals (left and right if the blades run up and down), in order to use with a two-pole 50-amp breaker (the norm). Things in your RV may use 120V only, but that doesn’t mean 240V isn’t present at the pedestal. If you’re checking voltages before plugging in, and you don’t see 240V between the X and Y slots in the diagram above, STOP!
Let’s talk about why. When you have a two-pole breaker (i.e. two “hot” wires coming out of it), the breaker trips if either leg exceeds 50A. The return path for any 120V loads is the neutral conductor. An RV with only 120V loads could have a total of 100A of 120V electricity flowing to loads without tripping the breaker. So long as it’s wired properly, with 240V between the two hot wires, the current carried by the neutral is well within its limits–carrying at most 50A, if all of the loads from your RV were powered by only one of the hot wires. The neutral current is the difference between the current on each of the phases–in other words, if you measured current on one of them (e.g. red wire), measured current on the other (black wire), and subtracted one from the other, that would match what you would measure on the neutral wire (white). You run the most stuff when it’s evenly balanced. Makes sense, right?
However, if you do not measure 240V between the two hot wires, those currents add, not subtract. Now, instead of nearly zero current on the neutral wire, you could have as much as 100A flowing through it before the breaker trips! Of course, that’s well beyond what a 6 AWG wire can safely carry. Assuming all of your loads are 120V, you may not notice a problem at first. If you’re not running large loads and never have a fault condition, you may never notice a problem. But an overload condition–or a short circuit–could go far longer before the breaker at the pedestal intervenes to protect the circuit.
You might have both air conditioners running, and be just fine (~15A each). You might have something cooking in your convection oven or microwave (another 12A or so). There might be something in the clothes dryer (~12A for a conventional 120V compact dryer). You’re over 50A, heating the neutral beyond what it can safely handle, but as far as the breaker is concerned, you’re a long way from it tripping. NOT GOOD!
Of course, as we’ve talked about in Power Adapters 101: What power adapter is safe to use?, it’s still possible to safely operate an RV with a 50-amp shore power cord with less than 50-amp 240V service. Let’s talk briefly about a few of those situations:
Adapters for a 50-amp RV to Connect to a 30, 20, or 15-amp Receptacle
These are all safe to use. While they leave you with only 120V power (and no voltage measured between the two hot legs), the smaller breaker size takes care of protecting the neutral. Yes, the current from each of your RV’s two legs are added on the neutral, but because the supply is 120V (a single-pole breaker), you’ll not have more than 30, 20, or 15 amps, respectively, from any of those supplies without the breaker tripping.
While safe to use, these adapters do mean that any 240V-only appliance will not function.
50-amp Receptacle Supplied by a Single Pole 50-amp Breaker
This is an unconventional setup, that’s not consistent with the NEC that’s adopted in most jurisdictions across the US. However, it is one that can be used safely. If you run across a 50-amp receptacle without 240V between hot legs AND it’s supplied by a single-pole breaker (i.e. the same width as a 15-amp breaker), I’d want to know who wired it, but after a little inspection (including checking that the wires supplying the pedestal are adequately sized) would be OK with connecting to it. Just know that you’ll have half the total power that would normally be available with a 50-amp hookup, or 60% more than would be available with a 30-amp connection.
Given what electrical codes use to calculate the sizing of feeders for RV pedestals, I’d have reservations about a non-standard configuration like this being properly supplied. Remember, the demand factors for an RV park are calculated based on the largest receptacle on the pedestal, not the combined loads of everything on there.