When I first got Callie I started off using a tote but it got annoying having to dig for everything so I bought a diaper bag that wasn't baby looking and then I got the pink version of this
Pet Travel Bag: Petmate Travel Bag for Dog Accessories
I keep dog food, medicine, extra blanket, extra leash, extra harness, extra clothing, if its cold a jacket, pop up dishes, cheerios, a comb, first aid kit, water, wipes, and nutrical. There may be more I just cant think well right now
. Some times Callie gets car sick although its not often anymore so I have a separate tote we call the vomit bag that has extra blankets, Clorox wipes, dog wipes, paper towels and plastic bags. Callie has gone on a trip where we drove from 9am to 6pm for three days only stopping for everyone to potty and to grab food and she was a puppy on that trip and did great we just look her for long walks when we where at the hotel at night plus to get her to eat we would make it a game at night. At least twice a year after that we would travel 5 hours to go see family with one potty break in the middle.