I think that gravity only would be impossible - somewhere there must be a pump.
for blackwater there is the heads pump that lifts the water to the holding tank height, from where it can be discharged by gravity.
For greywater they could go into the tank by gravity but from there you would have to pump them out.
Most natural position would be somewhere under the floor panels. Size, location and complexity of plumbing would depend heavily on intended use (kitchen, heads sink, shower basin) and intended period between discharges.