As Wundergore said, there is more to the equation than simply calories. In a survival situation you can feed your body crap 24/7 and get away with it, but before too long you'll start to become deficient. Sailors on long journeys were plagued with the Scurvy, a simple lack of vitamin C but quite devastating.
Eating an inordinate amount of carbs or saturated fats will offset weight loss to some extent, although I can't quantify this as I'm no expert. Some say that almost any carb intake is bad, such as the Atkins diet. My dad lost 40 pounds in 6 months on the Atkins diet, but he's over 50 and not very active. If you're running around alot of carbs are a good thing, to be sure, but if you're sedentary they will be converted to flab.
Some claim the Atkins diet is too much of a shock to the system and since more meat is consumed, one is at higher risk for heart disease. I am dubious of this claim, but for the simple purpose of calorie counting I avoid fatty foods.
As you might have guessed, the best solution IMO is a combination of things. I diversify my food intake (yes, my diet is PC) and eat a combination of carbs, protein and fats. Fats are extremely high in calories so if you're trying to stay under a daily limit, you'll have to avoid them. Two spoonfuls of peanut butter has as many calories as three apples, five carrots, or three slices of bread. The amount of salad dressing we usually use can triple the number of calories of the salad!
I do think that any survival kit should include a pack of multivitamins. The food itself should be extremely high in carbs, moderate in fat, and high in protein (10-15 grams a day).