Originally Posted by Viesel
... will it be difficult to change the rear bath to a center bath? ...
"Difficult" is relative. The task can get downright simple if you throw enough money at it.
For starters, your toilet dumps directly into the black tank. For various reasons, the exisiting black tank will probably not be usable with the toilet mounted anywhere else. So, you will have to design & fabicate a new black tank and associated plumbing.
Anything is doable, but for the effort level, it would be much easier & probably cheaper to buy & refurb an Airstream which already has a center bath as opposed to retrofitting the option to your '68.