My guess is they've become common in RVs because they do not take as much "east-west" room as a regular fold out sleeper sofa. Before RVs had slides and extra aisle space, most would not have had room for a traditional sleeper sofa. They are also lighter and can provide some storage space underneath.
The one in our MH does not have arms but is between backside of dinette and slide wall. When we open it up, I can lay on it and let me feet overhang past the slide wall to stretch out. My kids (who are really 20 & 22 year old young adults now) don't mind sleeping on it. DS says it is more comfortable than the overcab bunk.