Hi,
I think that what you saw was a pair of 30 amp plugs going into a 50 amp receptacle. While it might work in some situations, modern RV parks use GFI receptacles, and will not work with that adapter.
You might find a few RV parks with a pair of 30 amp receptacles at a RV site, however it would be a 'pair' of campsites sharing a single power post, with two 30 amp plugs - 1 for each RV. SO if the neighbor's site is filled, they will use on, and you will only be able to use one of the plugs, and not this adapter.
Any 20 amp GFI that you plug this adapter into will trip, because all of the power going out on the 'hot' line is not returning on the 'neutral' line.
At first I was thinking that you saw a used RV with twin power cords. While that is possible, the past owner might have installed 'anything' - but RIVA will not allow two power cords on one RV, so it was not installed at the factory that way.
Good luck!
Fred.