How to Winterize holding tank

[destination unknown]

In the past to winterize my holding tank I would do the following:

1. Pump out completly.
2. Add 3 times the normal amount of tank deoderant.
3. Pour 4 gals of antifreeze into bowl and from pump out line.

Been doing this for years but now towards the end of the summer, no matter what I do I still get a smell every time I flush the toilet.

Is there something I am missing?

Thanks