Can you really survive on nothing but potatoes and milk?

Yes, but then you'd die of molybdenum deficiency:

[I]f you're an active male between 19 and 30, of average height and weight, then one gallon of milk and eight pounds of potatoes will supply the RDA of most nutrients, falling a little short on the iron, folate, and niacin fronts, missing a lot of vitamin E, and striking out completely on molybdenum. Chug two gallons of milk with your spuds and all you're missing is about two-thirds of your vitamin E and, of course, your molybdenum. Not so nuts about milk? Fine, cut it down to a quart and choke down 14 pounds of potatoes instead. Now you're short on zinc, folate, niacin, vitamin E, and way low on vitamin A. And alas, still no molybdenum.

So what happens if you starve yourself of molybdenum? According to one nutritional reference book, "signs of molybdenum deficiency . . . are headache, rapid breathing and heart rate, nausea and vomiting, acute asthma attacks, visual problems, disorientation, and, finally, coma."