In the past we stayed for several months in the same RV park in our MH. We used Exodor Bio-Pak drop ins for the black and gray tanks. Sometimes there was a horrible eggy smell when we dumped the gray ...
I have had the tank smell only two times. Both times it was wen I was in a long term spot and let the tank build up to 3/4 before I emptied or when I did not dump after boon docking and waited till I got to the next campground. I put blue dawn dish detergent in it and put some water and use it and it is less and then put water and dawn in before I leave and drive and it is fine.