Congrats on finding a center bath, it's worth holding on to! There are many more options available with that floor plan than with the rear bath.
I had a bit of frame rot on my '73 Sovereign which required a bit of fabrication.
Rear cross member
Before After
Outriggers
Before After
Be careful about adding too much weight back there, as the shell actually helps support the frame.
Hang in there! Everything takes longer than you think, but it'll be worth it in the end (I think).