Raw Foods
Eating raw foods will cut out the need to cook as many meals, and is healthier in many cases. Cooked foods tend to destabilize enzymes, kill vitamins and antioxidants, and are more difficult to digest. Raw foods will include fruits, vegetables, nuts, seeds, extra virgin coconut or olive oil, and raw yogurt, to name a few.
To give you some extra space in your van, you can make fruit and veggie smoothies.
Tip: How to keep veggies fresh
Dense Foods
If you are really cramped for space, you may need to start eating foods that take up less space in your van or go shopping more frequently. Small or dense foods that are high in calories are typically going to be foods higher in fat. Living in nature will hopefully cause you to be more active so you can get away with eating a few more calories!
Here a list of (somewhat) healthy dense foods for a balanced diet: