Sewer smell from washing machine

I posted some time ago about a sewer smell. My washing machine dumps into the black tank. That is just fine with me. Out of nowhere a sewer smell developed. One day it is fine and the next a horrible smell. I exascerbated the smell by running the cab AC on recirulate. I figured that out and at least it was better. The washing machine is in the rear bedroom. We closed the door and sealed under with towels. When we opened it was a horrible smell. So we were pretty certain washing machine. Today I ran a gallon of antifreeze through. Then went for a ride. Smell is gone. Can only figure I lost the fluid in the P Trap. The day before the problem I used the tank cleaner a lot. I hooked a hose up to it and ran it for about five fills. I had time and a connection so wanted to get it very clean. I have no idea if that somehow created a draw that pulled the fluid out of the p trap or if totally unrelated. Early the day the smell arrived I was on a horrible road for awhile and certainly the back end was bouncing so maybe that caused fluid to depart. At any rate the problem is gone and pretty much certain it was a dry p trap. Anyone else had that problem? Just thought I would update.