Well, one of the benefits of the macerating toilet is that you do not need a straight down drop into the black tank. It pumps the waste through a flexible hose.
So, in theory, you could have the toilet toward the side of the rig, yet still mount the black tank along the centerline, if you were worried about weight distribution.
I believe some of the Leisure Travel Vans Unity models use a macerating toilet for this reason.
Also, the Thetford ones will include a set volume of water with each flush & fill. This can help ensure that you've got enough water going into the black tank, which could be useful if some in your party are prone to not using enough H2O.