Try diluted Ensure. One bottle in a water bottle, fill up with plain water. Take two swallows every 15 minutes, no more, then follow with 2 or more swallows of water. So take two bottles, one food, one plain. Refrigerate your bottles overnight. If that works, you can carry Ensure bottles with you for longer rides.
Don't wait until you're dehydrated to start drinking the food. Start after 1/2 hour.