There are many brands and kinds of holding tank additives all claiming to control odor and dissolve solids. Each one has it's proponents who will tell you it is the best. For me, all I can tell you is take your pick.

Many RVers recommend enzyme-based chemicals which use live bacteria to breakdown and digest the odor causing waste. I've had fair results with some of them but so far I have not found a standout favorite and none of these enzyme based additives does a really good job of controling odor.

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

I've tried a lot of home brew chemicals with varied success. None proved to work as well or were too much hassle. I've also tried some non-RV odor control chemicals.

For a while I was using OdoBan that I got at Sam's Club. It's a potent liquid that does eliminate the holding tank odor provided you start with a clean tank. But it's odor wasn't much better.

After all the different chemicals I've used I've gone back to the tried and true formaldehyde. Now before you get too excited, I know there is a lot of controversy on this subject. I'm not evangelizing but I don't care if you feel differently.

I've done my own research and I'm convinced that the whole formaldehyde scare is way too much hype with very little substance.

The following quote is from the Washington State Department of Transportation.

"Commercially available RV additives when used as recommended by the manufacturer will have no adverse effect on biological sewage treatment systems. "Home-made" mixtures of household cleaners and disinfectants should be discouraged as the proportion of each ingredient has not been established and the minimum quantity to control odors and biological activity are unknown. These "home-made" mixtures cause a temporary shock to biological sewage treatment units under certain conditions."

This quote isn't my only source but I thought I'd put one in just so you'd know that I did, in fact, have some credible information to support my opinion.

Please also note the part about using home-made additives.

Caring for your holding tanks.

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!

You may also want to read:
RV Gray Water and Black Water Holding Tank Basics
RV Waste Water System Care & Maintenance