You could have the dreaded poop pyramid and/or the smell could be coining from grey tanks.
I use Unique products in all my tanks except the freshwater tank. I just use vinegar and Dawn in it for rinsing it out. No problems yet but, before winterizing, I always travel with a little treated water in the tanks to slosh it around.
I learned this method from a video I saw on RV life. Seems to work well. There is also this:
After dumping and rinsing, I would keep 3 gallons or so of treated water in the black tank.