Feeling your pain
We have (had) the same problem on our Tuscany DX. Although we suspected the problem for over one year, we were not able to convince the dealer, until recently, when we were no longer able to empty the gray tank. The problem was verified when sewage backed up into the shower.
The dealer found the backwards connection and fixed it. We submitted the claim to Thor yesterday, and they denied the claim. This is a 2018 coach we have owned for 18 months.