Sewer odors

Pardon if this has been covered before but I found nothing in the search function. When dumping and then flushing the holding tank, we are getting serious sewer odors inside our 5er. This odor is really bad during the flush. We have tight connections and no leaks anywhere. We don't usually get odors inside if I do not flush the tank. I have water in the P-trap for the W/D connection. I can't figure where the odor is coming from or how to prevent it during the flush out.

Comments, suggestions?

Joe