The main thing I'd be looking for is the condition of the frame and the floor. While a rear bath may be more susceptible to rear end separation due to multiple sources of water damage (plumbing), a rear bedroom can have problems as well if the rear seal between the beltline and rear bumper fails. (I found out about that first hand with my '73 Sovereign
centerbath. The last frame crossmember was nearly gone from rust.)
The reinforcement plate just gives added strength the the fastening point of the shell to the frame. (A solid floor and rear channel support should be sufficient to hold the point together.)
I think the reinforcement plate was just extra protection.
But none of these things will prevent problems ultimately if axles are worn (rough ride, bottoming out) and running gear is out of balance (excessive vibration, accentuated at the end of the "see-saw").