The only foods I know of that are extremely high in potassium per calorie consumed are beet greens (not a generic term for other greens, but a very specific type of green) and celery.
Celery has about 100 mg potassium per medium stalk, which is only 6 calories. Of course, consuming say 20 stalks of celery to obtain an increase of 2000 mg in potassium intake isn't something that most want to do. It's potassium-dense in terms of calories, but not so much in terms of bulk.
Beet greens have about 1300 mg potassium per cup, and only about 40 calories, so that's pretty amazing. But they are hard to find.
Avocado is fairly high in potassium: 100 g has about 500 mg at a cost of about 167 calories.
Orange juice is fairly high in potassium but requiring substantial sugar intake to go along with it. Each 8 ounces has nearly 500 mg of potassium, to about 21 grams of sugars and about 110 calories. Still, calorie-wise, it's pretty dense in potassium.
An easy way to get potassium is to add "Salt Substitute" (potassium chloride, KCl) to food or water. If I recall correctly, each 1/4 tsp contains about 600 mg potassium. When dieting, depending on the rest of the diet I will add KCl to a gallon of water.
Also, for some reason nutrition information on foods and supplement labels sometimes omits substances which in fact are present in substantial quantity.
The two main examples of this seem to be potassium and phosphorus. Foods or supplements may contain these, yet give no listing for potassium and/or phosphorus at all. Of course that means you can't count on potassium being there, but it also means that you may have been getting more potassium than you've realized if you've tried analyzing available nutritional information for what you've been consuming.
For example, if you've done fine on the V-Diet and are now wondering how you survived with zero potassium, the answer is, actually you were getting quite a bit, though I would suggest supplementing it with another couple grams a day intake from KCl.
The value I've seen for the RDA is 3500 mg/day, not the higher figure quoted about, though I think a lifter will do better with more like 4500-5000 mg/day as personal opinion (with probably some dependence on weight -- I have in mind someone around 200-230 for that figure -- and perhaps some dependence on amount of sweating.)