Purchase a bus from an auction. Paint the exterior to add a personal touch. Remove the entire interior from the bus. Take complete measurement of the floor space, and then sketch your plan of what you actually desire to have in the bus. After creating your plan, bolt the floor every two feet from floor to window with 2X4s, as it will be the interior studs for all of your equipment that you will be going to install. Install insulation around all sides of the bus and across the ceiling as well, it will help you controlling the temperature and also soundproof your RV.
Now frame your sleeping room and front divisions with 2x4s. Use plywood and panelling to create walls between each division. After you are done with creating rooms, create a framework of 2x4s and slats according to the size of the beds. Bolt them around the sides and floor of the bus and place mattress over it.
Whatever type of wood is available easily and suits your taste as well, make cabinets with them and hang them on the walls. Only Pocket doors should be installed in your RV, as they occupy less space, and can easily be installed.
After you are done with other living rooms, install the portable toilet in that area you have determined for bathroom. Install the water tank under the bus. Attach all the plumbing pipes with your installed sink and shower stall.
Last but not the least, add the couches and chairs in your RV. Bolt them all to the floor in order to prevent them from sliding across during travelling.