If you noticed that prior to buying the trailer would it have prevented you from buying it?
What would you like them to do ?
OK, they admit it is a design problem, but it would be absolutely impossible to correct it with out moving the under floor connection to the black water tank, and then doing some work to the floor.
There are several good suggestions here on a quick fix.
Just use one of them.
When we bought our trailer, (new) we found all kinds of tid bits that we didn't like, but the trailer had the exact layout and size that we wanted, so we just corrected things and added things and now we wouldn't part with this trailer for anything
Jack L