I have a 1969 31ft rear bath and suggest you do a search on "rear end sag" or "rear end droop." Personally, if I had it to do over again, I would probably keep looking for a mid bath unit. The rear bath is a great location for it, but unfortunately the extra weight and a multitude of other factors cause the rear floor to rot and sag. It's not just an appearance problem but can cause damage further forward on the trailer too. I have done some minor retro fitting, but the only cure "they say" is replace the back 8ft or so of floor and some of the metal. Many variations on the problem, based upon year, model, etc, but the result is similar.
best wishes, bill b.
I still love our 31 footer tho, it's like a troublesome child. It's worth the effort to keep it on the road.
1962 Airstream Tradewind
2001 Ford 7.3 Diesel