Mountain Dew is often described as 'citrus' flavor but that is a broad range of fruits with infinite possible combinations. It does have orange juice in the ingredients, but also "natural flavor". How can I recreate the flavor of Mountain Dew using ingredients available to me, such as fruit juices, with no added sugar or caffeine? 
OK, this is dangerously close to a recipe request, but far enough away that I feel OK to answer.  Use the rule of two.   2 cups of sugar   2 two oranges   2 limes   2 lemons     Juice and zest the whole fruit.  Strain and and fill to 1 gallon with water. Filter if you want it to be clear.  Carbonate using whatever technique you normally use whether that's a Cornelius keg or soda stream. 