The gray tank commonly smells worst that the black. That is just a fact of life.
In 40+ years of camping I have never treated either tank. I don't treat the sewer lines in my house either. There is no reason to treat a tank other than to spend money it accomplishes nothing.
Now some may take exception to this, but I doubt they will have any scientific statements to support their point.
WBCCI 12156 AIR 3144 WACHUNG TAC NJ6
2004 Excursion 4x4
1991 34 ft. Excella +220,000 miles, new laminated flooring, new upholstery, new 3200 lbs axles