I have had 3456rl since March. Lived in it for 3 months. I olumbed the 2 grey tanks together. Had to out soray foam around roof vent to black tank as anytime the middke ac ran we had a smell. Worked great. Had to rewire the televisions as it was not done correctly. We love unit though. I never bring to dealer if I can avoid it.
Saw your comment in an old thread and trying to figure this same issue out you mention.
(Cant give URL in msg as it restricts that)
I have a 2017 3456RL and been smelling fumes as well similar to you, its when we run the roof vent fan in galley or even bathroom and everything is closed up, we get that smell from black tank. If we open a window or door it stops.