Ditto on adding a few gallons before each use of the tank. Also don't skimp on water when flushing each time. Trying to conserve water only makes it worse. Then fill the tank to as full as possible before opening the dump valve.
You need to rinse the tank out before leaving a CG using an in-tank rinser, down the toilet wand or back flush elbow. In some cases you may need to repeat the fill & dump process again.
Treatment chemicals are a waste of money. My lovely DW is into selling Avon products and now claims Avon shampoo works great. Whatever... :M If you need a really deep cleaning, liquid Drano can be used. Won't hurt metal or plastic and our owners manual says to use it. Don't rely on the holding tank indicator panel because the sensors get gummed up with slime over them. False readings are normal. Better to peer down the toilet with a flashlight.
TP doesn't need to be the "for RV use" labeled stuff with it's mafia-inspired pricing. There are a few brands of household TP that disintegrate the same. Costco's Kirkland brand is what we use. You can do a test in a jar with a couple of sheets to check.