you should not need any artificial additions to either your gray or black tanks.
I have ceased using chemicals in either my black or gray tank and I have never had an odor problem. The chemicals I have are left over from the recirculating toilet in my prior SOB trailer; that is a syatem that needs lots of chemicals.
My trailer is in a storage lot and I often use the toilet (liquid) several times between trips. Even with sitting used for a week, I have never had an odor except once when the vent fan was on, the door was closed, and TP had wadded up so that the dump valve didn't close completely.