Ther are More than enough drive over dump points on the continent almos every services has one.Some of the coaches that go over to the continent definitely open the dump valve on purpose to empty the tank. A couple of years ago son in law and family did a package holiday that included express coach there and back, tank was dropped more than once each way and the drivers told him it was normal practice. I don’t believe they did it when in the U.K. but that’s probably because it didn’t need it rather than any other reason
In my 30 years motorhomeing in Europe I have never seen that practice.
And have been on a good few coach trips to Spain Germany France and never ever seen any couches dump black waste any whare it should not have been dumped.
Wildbill