Well, fancy meeting you here!I basically don't even know where to start.
Well, fancy meeting you here!
The first thing that you need to consider is where you will place your potable water, gray water, and black water tanks. Once you know where you have room to mount those you will also be able to figure out how large the tanks can be. I would start by spending time under the bus looking at the frame rails and such to figure out if there is a way that the tanks could be mounted there. Since water is about 8# per gallon, large tanks can be very heavy when full so they need solid support and mounting them low will help with handling by lowering the center of gravity. Once you find a place to locate the tanks you need to figure out where the plumbing for the drains into and from the tanks can go. All of that is very important to do before you start putting things into the interior. As you start to design the interior you need to keep in mind where the water lines will go. It is very common to put the fresh water tank under the bed as that keeps it inside out of the cold and the pump can be mounted next to it with easy access. At some point you need to add in the water heater to the mix, the sinks, and the shower. The toilet is usually mounted directly above the black tank so that waste can drop directly into it and eliminate the plumbing between. It will need a fresh water supply. When I helped a friend convert an old school bus years ago, those were the things we addressed first. Not far behind should be a location for an ASME propane tank which you could get from Amazon. If you keep the water heater, furnace, and stove in close proximity it will make the propane lines much shorter and more easily hidden.