I can't tell you how to plumb the bus, but can offer a few tid-bit's of infomation.
First of all, PVC piping is not the best route to take, you have to buy all kinds of fittings, elbows, glue & primer etc. You'd be far ahead of the game if you buy a couple of 25' coils of PEX tubing (get one clear for cold water and one red for hot water, sold at Lowes etc)... You'll still need wom elbows and fittings, but less of them and it'll be much easier to install as this stuff is far more flexible than PVC, which means you can make curves etc without cutting and glueing fittings, elbows will still be needed for sharp corners. A simple PEX crimp tool & crimp rings makes sealing them easy. Use 1/2 inch...
You really should use seperate tanks for black & gray. 2 reasons come to mine...first is back draft...if you wind up with a dry P-trap oders from the black tank can permeate into the cabin, this can be a really bad problem while you are driving down the road. The other is, many campgrounds will NOT let you in with one combined tank.
You can buy some very affordable tanks on ebay from a power seller called Tri-State Rock Your World
they have 36 gal waste tanks for $45 each (I bought a pair of these from this seller), he also has fresh water tanks. Also on his site are Rubber Tank Grommets, and I highly recommend you get some of these. Use a hole saw to cut the hole in the top of the tank at desired location for pipes to go in, use these grommets to hold & seal the pipe entry. If you cut the right size hole, these things slip into place, then the pipe just slips into them. Each holding tank will need waste input (usually 3" for black tank and 1.5" for gray), you will also need 1.5" vent pipe to go up to the roof for proper tank ventilation (this vent pipe can be shared by 2 tanks though I recommend seperate vents), each of them will require a vent cap. These waste tanks I listed come with outlet fittings, so you just have to prime and glue the desired outlet fittings into the drain hole. They will need to pipe together to a common outlet, each controlled by seperate dump valves. LIKE THIS
Highly recommend you use appropriate RV type toilet, these have gated valves and stay closed...esepecially when on the road. House type toilets use too much water, and when moving the water in the trap can shift and go down the drain and then you'll get oders wafting in from the tank.
I also recommend a GREAT Ebay power seller called TRI State Surplus
as he has lots of good stuff and the best prices anywhwere. Book mark this guy and check back often, he changes/adds inventory daily!! I have bought from this particular seller over a dozen times now!