If your tank hasn't been cleaned thoroughly and regularly, it may need a heavy duty cleaning. Our owners manual says to use Liquid Drano. I've used it once or twice a year to get the black tank back to factory fresh, factory clean condition. It won't hurt metal or plastic. (Don't dump it into an on-site septic system.) Some won't use it though for fear of hurting seals. After 4 seasons, no problem for us.
There are commercial tank cleaning chemicals available. I think Thetford has one. It needs to be above a certain temp., then you add some water and drive around to let it slosh around. The "Geo method" is another option (google it for details).
After cleaning the tank by the method of your choice, as mentioned above always put in a few gallons before each use of the tank. Then don't skimp on water each time you flush. When ready to dump, fill the tank to full or near full before pulling the dump valve. If you have an in-tank rinser, let it run 10-15 min. while breaking camp. Or try a back-flush elbow (won't always work depending on tank configuration and outlet location). In-tank rinsers can be added if you have access to the tank.
If your toilet drops down vertically into the tank, you can shut off the water and peer in with a flashlight to see how clean it is. Keeping the tank as clean as you can may also help make the sensors indicate more accurately. I'm a non-believer in tank treaments that you drop in each time you dump the tank. If the tank is kept clean, the AAV hasn't failed and tank vent pipe is functioning right, you shouldn't need treatments. Turning the fan off can help.
A failed air admittance valve under the sink is a common cause of a bad odor. Also, if you haven't used the shower for quite a while, the P-trap could be dry and be letting odors in.
