One reason Airstream originally put the bathroom of your trailer in the rear was to decrease the tongue wright. So, if you want to move it to midships over the axle (and presumably move a bedroom to the rear?), you'll want to consider what that will do to your tongue weight as well as overall weight.
Another consideration is ease of plumbing and black/gray tank placement. You want to have a "direct drop" from toilet to black tank, and from black tank to the point of sewer exit. The gray tank drain lines and point of exit can have some bends as long as everything still flows downhill.
I am pretty sure others have made this modification though, and here are a couple of threads with some discussion about it: