My hanging bag lost it's cord so now it sits on the floor in the back of my car. I use it to throw out trash and then I'll empty it at a gas station in one of their trash bins.
I have a small square container, with small weights on each side for balance, on the middle of the floor in the back seat. Just a reach back to deposit trash paper and that. Empty about once a week.
My new resolution with the new van is no trash allowed. I hated my old van so much that I didn't care if trash was in it. But with the new van it is so clean and pretty and I don't want any trash in it. I have a little pouch bag on my shift. It's small so it makes me have to clean it out often, luckily we have garbage cans next to the mailboxes.
We use the bags that newspapers come in for garbage and we keep it on the floor on the passenger side. It's narrow so garbage doesn't fall out of it easily and since we have lots of newspaper bags we can toss a full one and grab another one.
I always keep a box of kleenex in my vehicle and usually end up using the empty box to stuff garbage into (small things like slips of paper and kleenex), when it gets full I toss it. By then the next box of kleenex is usually empty and I have a new garbage box.
If we grab something from a drive-thru all the garbage goes back into the bag it came in and it gets taken out at the next stop we make (either at home or someplace I can find a garbage can easily). For long trips I make sure to slip in several grocery bags to collect everything and try to drop them off whenever we make a stop somewhere.