I start off with a few gallons of water and a half cap of liquid Tide.
Every time we do a #2 we add a toilet bowl full of water.
We have never had a tank smell problem in 14yrs of camping.
Being weekend campers and fulltime employed the camper may sit for a few months between uses. The only time we experienced a smell issue is when the shower 'P' trap dried from lack of use and gray tank gasses came in from there. That smelled as bad if not worse than anything that goes into the black tank.
Use your toilet, enjoy your toilet, learn how to maintain your toilet. Lots of water.
TIP: Prior to sitting down for a #2 cover the water in your bowl with a layer of toilet paper. No marks are left behind after the flush.