The real answer to your question really depends on what kind of a setup you have. If you have Sani-Flush (like I do) all you have to do is let it run for about 5 minutes, put a bit of chemical in and add a bit of water. (I was stunned that there was absolutely no odor when I had to change the seal on our toilet despite the fact or rig had been standing for a month. I did not even have to cap it while I was waiting for the parts to arrive from a mail order house.) Obviously, this method works.
I add some Thetford gray water treatment for the other tank. I have never had a problem with the tank making an odor. (The washing machine - yes - but that is another topic.)
This works for me.