I have always used a travel size toothpaste and travel tooth brush...can often be bought together and weigh very little.
For flossing I tend to use things like the "Reach" which is much easier to use than straight floss, and keeps your hands out of your mouth. One bow can last quite a while if you rinse it and let it dry.
For spitting I either use a hole dug for the purpose, a hole dug for food bits when cleaning the pots, or I clean my teeth before using a cat hole or spit into the campfire ring if there is one a the campsite. You don't need to use very much toothpaste but in theory spitting around the campsite may attract critters to it so its best to bury it.
I know some make their own "toothpaste" but you have to research that carefully because getting it wrong can damage your teeth. Personally I have more interesting things to think about.