Strictly a matter of personal preference and your planned use. If you will have others traveling with you - kids or friends - then yes, it is ofetn convenient to have a bathroom that is separate from the master bedroom, either closed off by a separate door or located at the other end of the RV. If it's just you and your partner, it's probably most convenient to have the bath adjacent to or right in the bedroom.
Interesting question, thanks. I have wondered about this also as we like the idea of two separate toilet areas with separate sinks. Would the bedroom get less noise from the living area, if the bathroom were between the two? Would the motion of the back of the coach affect anything in the bathroom (plumbing, personal items)?
I have a Prowler TT with the bath at the rear. The motion does not affect it all that much. The use of spring loaded retaining rods in the medicine cabinet is advisable to avoid a rain of pill bottles when first open it after moving is recommended but that is about all. The pantries and pot and pan storage in a kitchen makes it a pita in the rear. Bathrooms have fewer things to tie down.