I've done it, too. Two cats, one of whom would relinquish the litter box only under extreme duress (the other cat's eyeballs were floating and the thing that had died in its gut was desperate to come out), in an '82 Subaru wagon that was packed to the gills. Oh, and it was August. And the car had old-fashioned roll 'em down windows. And we could only roll down the front two because the car really was packed to the gills. The stench was powerful. It crept and fungated. We tried not to breathe as we laughed. Egads, what an odor!
Two adults, two children, and two Siamese cats, in a Datsun 240Z. From Texas to Rhode Island. And my parents smoked then.
If that wasn't the worst move, it must have been the one from San Diego to Texas, driving across the desert with hour after hour of the Watergate hearings on the radio.