We emptied water where the gas station operator and chemical toilet explained to us as well. we fill water. They are € 3 the service or free if you consume at the gas station.

Just ask the gas station and they kindly show you the place where you can dump everything, including a chemical toilet, into a space under the steel plate in a bus depot. Water available for rinsing. Not very handy at all but it works.

Not self service as the disposal point is up a ramp behind the petrol station inside a large bus garage. But the friendly staff very happy to show you where to go. Disposal is under a large steel plate in the garage floor. Water available but we did not use it.