Usually while driving, but sometimes when parked, we would get a bad sewer smell. First we closed off all the little breather holes in the vent pipes under the sink - helped. Then we started pouring water into all the drains to be sure enough fluid was in the p-traps to seal the smell out.
However, couldn't get to the washer p-trap!
Realized a good hard plastic hose accessible from the door area that goes directly into the p-trap was the solution. Now, we just open the closet door (Full of clothes, of course!) and use a little funnel to pour a glassful of water down the tube into the washer p-trap - no more smell! Yeah!