(pick this one for gravity fill only or this one for gravity fill and city water hookup) , a large fresh water tank (my current favorite for a large supply), supply piping (preferably PEX), a 12v water pump, your fixtures and their valves and attachments, your waste piping (perhaps PVC), and a gray or black water tank with a waste valve.
However if you will be parked at length next to a municipal water supply or at an RV park you could actually hook up your bus to city water which will come into the bus pressurized, bypassing your tank and pump, and this will save you the hassle of filling your tank when itโs empty and using a noisy pump to push your water to your fixtures.
run some PEX off of the vinyl line attached to your water pump you should install an accumulator tank which is a little pressurized vessel which will assist the pump with water flow and prevent it from cycling so frequently.
From there you will install a tee to supply your water heater, and along with the hot water supply line from your water heater you can run PEX lines to your future sinks and shower, and anything else you need hot or cold water for.