I'm planning on putting a dedicated grey tank on my bus at some point. I'm planning on parking on private land over the fall/winter, so if I don't have the grey tank by then it'll just go onto the ground. That wouldn't be okay anywhere else, but since I'm on my own land I'm fine with it.
I've seen someone do a combined grey/black tank before. I'm not too sure about that. If it got full it could back up into the shower. I don't want poop in my shower.
I'm thinking of getting a 40 gallon fresh water tank, 40 gallon grey water tank, and 20 gallon black (poop) water tank. I think that's pretty standard on a lot of RVs. It may even be a little big.
Many campgrounds have a waste dump station. Sometimes rest stops on the highways will have a dump station as well. Depending on the rules where you live, you may be able to dump grey water on the ground. Many people save the grey water so they can flush out the hose they use to dump the poop water.
You can find fresh water in a lot of camp grounds too. I've been to one that had a water hose at every other space for filling your fresh tanks.
As for the toilet, I'm thinking of going with an RV toilet. You can pick one up at an RV supply store or online. I've heard someone on here say they liked the Dometic 310. I might buy that one. When I poop in an RV toilet I like to lay a sheet or two of paper in the toilet first. It helps keep the bowl clean.