I get contractor garbage bags from the hardware store (Walmart’s are ok but not as heavy duty), and stick them inside the “Trash-A-Roo”, or just tie a knot in the bag and strap it to my roof rack if I have several.
As for dumping trash, it depends on; where I am compared to home, how much trash I have, and how much more road time I have. My preferred dumping place is at my local waste management, because they give a discount for the weight when you are returning from the woods (and even take it for free if you bring a Forest Service Volunteer Form with you). Second best place is the dumpster in front of my house... out on the roads, I look for dumpster that are easy to access at the truck stops and local gas stations... Some larger parks have trash facilities as do most restaurants stops... so lots of places to dump your garbage while out.