Foul Odor from Toilet
What is put into your tanks does smell bad, and no home remedy or commercial product will do much to help if your tank vents are not clear, and it sounds like that may well be the problem. Each holding tank has a pvc pipe that runs from the RV roof down into the tank to vent the air from the tank. On the roof there is a cap over that vent to prevent the entry of rain into the tank. There are several things that can be a problem with the vents. I would remove the cover from each vent and first use a garden hose to run water into it to make sure that the vent pipe is open all of the way to the tank. Bugs sometimes use the vents to make nests in and plug them up. If they are not clear, gently run some stiff hose or plastic of some type through them to clear any obstructions. Once you know that they are clear, check to be sure that the vent pipe has no gaps that would allow the odors from the vent to enter into the RV around the top of the pipe. If they do, plug that area with caulk to seal it. When you put the vent caps back on, you can also replace them with vent covers that are a venturi that will keep a slight vacume in the tank any time you travel or if there is any wind. One version even revolbes with the wind to change the direction it points to work best. I don't have those but have heard really good reports on them.
According to a seminar that was put on by Thetford, manufacturer of RV toilets and sewer system equipment, the best way to clean the tanks is to fill about half full of water and then use the detergent, trisodium phosphate (TSP) that can be found in the wallpaper department of any home store. Mix the TSP in a gallon or two of water using the amount that the directions call for for the amount of water that you put into the tanks. Once you do this, drive the RV around for a while, where you make turns, stop & start and such to agitate the mixture. It would not hurt to let it sit for a day with the detergent in the tanks and then do it a second time just before you dump. That should remove any built up residue unless it is very excessive and thus allow you to start with a fairly clean tank.
Also, as mentioned above, most toilets do have an over-flow that has a P trap in them. If that P trap is empty it will vent into the RV. To be sure that it is full of water, just fill the toilet with water until some water goes down that over-flow.
If you tank vents work properly and if the tanks are fairly free of debris, you should not need to add anything to them except in very hot weather. As long as you do not introduce some chemical into the tank that will kill the natural bacteria, it will break down naturally and become a liquid as long as you use plenty of water. You will find that most of us who are fulltime use lots of water in our tanks and very seldom anything else. But since your RV is used, the previous owner may have left a lot sticking to the inside of the tank so I would do both of these things first.
You will find that there are many different opinions on what works best, but these are the things that an RV shop would do first.