There are many brands and kinds of holding tank chemicals all claiming to control odor and dissolve solids. Except for avoiding those containing formaldehyde or similar take your pick. I have even used liquid laundry detergent with fair results.

Some RVers are recommending enzyme-based chemicals which use live bacteria to breakdown and digest the odor causing waste. We've tried these and they do seem to work. I've had fair results with TST brand from Camco but so far but I'll use any chemical that's on sale. I have a thing against putting money down a black hole.

I don't think there is any difference between the liquid and dry as far as how they work. We like the dry packets just because they store better but they also create dust when poured which we don't like to breathe.

For the last year or so I've been using OdoBan that I get at Sams Club. It's a potent liquid that really does eliminate the odor provided you start with a clean tank.

Generally, you need to add chemicals to the black-water tank after each dump. Same for the gray-water tank although I don't feel it is as important.

After you have drained and rinsed the black-water tank, close the valve and add enough fresh water to cover the bottom about an inch deep. Then add the chemicals. I like to add the chemicals a little at a time as I 'flush' the toilet. I think it helps disburse the chemicals.

When we're hooked up to park services, we leave our gray water tank valve open. When we're 'hooked up', for and extended time... several weeks... I like to close the gray water valve, add some chemical to the tank, and let it fill up. I do this about once a week. I seems to keep the tank from getting sour... which can be almost as bad a smell as the black tank!