I think it's a vent issue. You're dumping and that can cause the water in the "p" traps to be drained...allowing odors into your rig. More with the black tank than the gray I would guess. I put about 3" of water in the bathroom sink in our rig before we dump. That way, when we're done dumping, I just pull the stopper and put water back into the sink's drain.
I make sure I turn off all the ceiling vents but opening a bathroom window when you are putting a hose down the toilet to hose out the tank will bring up some bad odors.
Check under the kitchen sink - some rigs have a vent pipe there.
Check the vent on the roof, make sure that's not clogged.
That's about all the suggestions I can come up with from my experience and what I've read on this forum from others.