We did it! The trailer/cooler chest combo worked beautifully to keep the ground beef and dairy products we purchased cold from store to home (7.5 miles). And we still had room for Pepper to ride comfortably so I could take him for a visit to Petsmart, which is in the same complex as the store.

THANK YOU EVERYONE for your wonderful ideas and suggestions!

Edit: This is going to work perfectly for our frozen stuff!