Both my fresh and grey water tanks are outside the bus. There is a socket for a whale submersible pump on the driver's side, so I have to take the fresh container from under the sink and attach it when its needed. Pain when it's raining cats and dogs.
For waste water we use a collapsible bucket that we place under the bus next to the fresh tank, it is piped through the floor.
If I didn't have a welded on belly pan I'd probably look into relocating both of these underneath.