Ditto on what Karen said. The discussion of treatments vs water only are very popular on every forum I have visited. The can get quite animated as proponents on each side try to convince the other that they are full of ....!
We are currently non-mobile and so do not get the sloshing action either. We do NOT use any chemical products. We DO put the tissue in the tank. We do have tank sprayers but I do not use them each time I empty.
When the tank is ready to empty, it will be very obvious when flushing. The flushing sound changes as the space in the tank fills up and the water line gets closer to the top of the tank. We do not flush until the sound changes so we can get the most water movement as possible in order to move the solids. Also, the longer we can delay the longer for the solids to break apart.
Note that I said "break apart". My reading and experience does not indicate that the solids are broken down or biodegrade as they do in a septic tank because they are not stored long enough for the bacterias to do their thing. These are called Holding Tanks, not septic tanks. But, they will break apart if they are submerged for a while.
Periodically, perhaps every couple months, I use a wand on a hose to spray out the tank.
I turn off the water to the rig. Run a separate hose with the wand directly from the water connection outside into the bathroom. Push down on the toilet flush valve and stick the wand down. I turn on the water for the wand and spend a good 10 or so minutes directing the strong spray around the inside of the tank.
I marked the wand so I know which way the water is spraying because I cannot see the nozzle when it is down below. I find this very helpful. It helps me control the direction of the spray and ensure all parts of the tank are cleaned. Also, be sure to direct the spray up and down, not just on the top or bottom of the tank.
After you do this a couple times you will become very aware of the piping from your toilet to the tank and be able to get the tank very clean.
When the weather is very hot you will get a little odor when flushing. This is normal as you are opening a hole directly into a tank containing some very odiferous things. Warming things up increases that. Also, if you have an exhaust fan going, it can pull a little bit of odor from the tank. Although this is not pleasant, it is normal. If you get a LOT of odor, or if it lingers, you may have a blocked vent as Linda pointed out.
On an aside note, if you get some bad odors be aware that your GRAY tank can be very smelly. Also, if your rig sits for long stretches without water being introduced to the drains, the water in the P trap can evaporate which opens a pipe directly to your tank. Simply pour some water down the drain to fill this back up which reinstates the odor barrier.
I get a little windy with my responses sometimes but I want to share as information as so many folks graciously shared with me when I first got started.