I use a Sea to Summit dry bag. It has very thick plastic and is completely watertight. You can seal it up so it doesn't stink up the car and you can even store human waste and the smell is contained. You can hose it out and it is not affected by clorox.
The one I'm using is about 2/3 the size of a kitchen garbage bag and seems to be just right for two people for a few days (I burn anything burnable).
I hang the dry bag off the rear mounted spare tire while I'm on the trail. I'll throw an extra bungee cord across it if I'm about to hit the highway. You could also carry it inside your rig - the smell is held in completely when it's sealed.
I wrap any sharp items like can lids inside other trash so I don't tear the bag. If the bag stinks after emptying, I hose it out, then put some clorox in there and seal it up and shake, then hose it out again - it's good as new.