We have travelled quite a lot with a black tank----and providing we carried a macerator for those higher than ground level dumps which are too common in Europe, dumping is no problem.
Most---not all---- people use chemical in their cassettes/porta-potties and this should never be put in a septic tank----------------we never used chemical in our black tanks, and the cassette system that we now have uses a 'sog' ventilation device and also does not require any chemical.---------even though we have two cassettes, we prefer the extended visits to dump, and are thinking of changing our van's cassette to a 'proper' toilet.
Nigel & Pamala----- UK