One reason I put my bathroom and all my tanks behind the rear axle was to stay away from that driveshaft, and to keep the tanks and the plumbing tucked up high. Note that the drift shaft moves around as the rear axle moves; it would suck really badly to have that driveshaft somehow knock out your black plumbing. Also, unless you put a internal valve between your black tank and the cross-pipe, that cross pipe will always be filled with the nasty stuff. So anything that happens is going to cause a leak with the gross stuff in it.
Here is how I have my bus drain plumbed currently:
This photo is taken from behind the axle, from the driver's side.
The bus under body is divided into 3 "Sections" by the two frame rails. There is a center section, between the rails, and then there are drivers-side and passenger-side outboard sections.
The 25gal black tank mounted directly under the toilet, in the drivers-side outboard section of the underbody. Goes immediately into 3" PVC elbows into Valtera 3" + 1.5" double black+grey blade valve. Note that the valve bodies are mounted upwards at a 45 degree angle. I read a tip that mounting them straight horizontally means that crud collects in the bottom track that the blade rides in, which can lead to leaks or other problems.
Note that because of the double-valve assembly, my dump fitting actually rotates freely up and down. So when I am under-way, I tuck it up inside the bus. This photo is a bit deceptive; very little of the plumbing is visible from outside the bus.
The gray tank is mounted in the center section, and has a 3" -> 1.5" reducer. 1.5" PVC slopes down and crosses under the frame rail to tie into the valtera dump assembly.
Ideally, my grey drain should also be 3" dia throughout, but in the interest of time and using the pre-made valtera doulbe-gate assembly, I have it plumbed using 1.5 for now.
The reason to use 3" dump plumbing everywhere is so that when you dump your gray AFTER dumping your black, you can completely fill the pipes and the drain hose that just had your 3" of black stuff going through it. This helps clean out your dump hose and pluming much better.
I'd definitely run separate black and gray tanks. I found that my gray tank filled quickly, and once I over-filled it. I was happy that dishwater & hand-washing water was the only thing leaking out of the vent pipe in that tank. A lot nicer than blackwater leaking out of anywhere.
My basic idea is this: I do not ever want to do any maintenance or road-side repair of anything related to human poop, nor do I want to have "performance problems" or "quirks" in a system designed to keep my family's poop away from my family. So I adopted a "take no chances" approach to engineering the toilet and black system.
PS: I highly recommend the Wastemaster kit from DrainMaster. It is the cam-loc dump fitting you can see in the photo, and a special dump hose that is truly awesome to use.