Do a search of restaurants and natural food stores near where you'll be staying. Or even map out your road trip route. By doing this, you'll be able to adjust your travel accordingly based on availability of the best food. I use two resources in particular. The first is called the Eat Well Guide. It allows you to search for your dietary preference by entering a key word. It will then find matches for you based on your whereabouts (just enter a city and state or a local zip code). Also, it features a handy trip planner that will automatically map out the best places to fuel up along the way.
The other is called Happy Cow. It, too, has a searchable feature, allowing you to find nearby restaurants and cafes with plant-based menus, anywhere in the world, making it particularly good for international travelers.