In the 50s, rear-bedroom/center-baths were very common. But the baths were typically smaller wet-baths (or, as a friend of mine calls them, "shoilets").
One explanation I've heard for moving the bath to the rear, was that it put more weight at the back to create less tongue weight at the front. Of course, the additional weight also seems to play at least some part in rear-end separation that became common on certain vintage units, especially the longer ones.
As far as which is better (and structural issues aside), it just depends on how you're going to use the trailer. I like the rear bath because it is spacious, and I like having a full tub. Other folks like the center bath because it allows for a rear bedroom, which is pretty desirable, and it also doesn't force all restroom traffic through the bedroom.