If it is a '50 Ampere' R V
There are FOUR wires from the receptacle to the R V
One of these is a GROUND, and is not normally in use
(only in case of fault......trouble)
One of the others is a NEUTRAL, which is a return path
for the current supplied by the TWO HOT wires I will describe next
(Yes, the current reverses 60 times per second but that
can be ignored in this discussion)
The OTHER TWO WIRES are HOTS......you can measure
220 Volts between them.....but, your R V probably
does not use 220 volts,
You can measure 110 volts
between EITHER of the two HOTS
and NEUTRAL. Your R V uses these.....the loads are connected
between either ONE of the two HOTS and the NEUTRAL. To get technical,
the voltages are out of phase and only the difference in current between
the two HOTS is 'returned' by the neutral.
So, you plug into 220 volts, but USE only the 110 volts
between one of the two HOTS and the NEUTRAL.