Early in the project, I was comtemplating 55 gallon barrels.
I had 300 or more plastic ones at the time and it seemed like a good idea.
They were just a little too big around and stuck out the bottom of the skirt about a foot.
This killed my ground clearance and I wanted to be able to go on somewhat unimproved roads......
The other problem with barrels is mounting those round suckers securely.
I used one of the cassette toilets for a little while and simply hated it - it was just "nasty".
I found the thetford sitting over the black tank much "nicer" to deal with.
I also included a flush fitting in the back of the tank so I can simply hook up a hose and flush the tank from outside the bus and not have to carry a hose inside and flush thru the toilet.
You could use a barrel for freshwater on the inside, but it will take up a lot of space and the space around the round profile is wasted.
You might try here: http://www.tankdepot.com
That's where I got my tanks and they have tons of different shapes and sizes. Just measure your area and grab tanks that fit.
While not cheap, the finished product works and fits well and is proven.
I think I had about 650 in all three tanks and rubber grommets to fit them.
Add in another 75 for dump valves, etc.
Hope this helps!