Interesting you should ask this question at this time because we just went through a significant "episode" with our washer on our last trip. I did all my normal "tricks" including water down the drain pipe and water down the vacuum break pipe. (I have a funnel permanently placed the drain pipe to add water from time to time to keep the "P" trap full.) But, this time nothing seemed to work for more than a day. I even added the gray water deodorizer to the water with no success.
Our solution, that seems to be still working, was to run the washer through a quick rinse/spin cycle then add about a half cup of bleach in the bleach tray and run it through a second wash/rinse/spin cycle. Apparently our unit had something in the washer itself that was causing the odor and this was the only way to flush it out.
We made it two days in Benson, AZ, through Yuma (in 107 heat) for a day and into So. Cal. with no additional smell whatsoever. This is our first flare-up like this in about three years. At least in our case, just adding water to the drain tube is not always enough to solve the problem.