I believe it does dispense all elements since I got a fire assault rifle, corrosive sniper rifle, and a shock rocket launcher. I'm not sure about slag, but I only did open the chest three times. Every time, I got good stuff (or my equipment was just crap).
Like I said before, I opened the chest three times, although I guess some might even call that too much (I'm a level 26 Zer0)
Those are my two cents... Use them as you wish.
EDIT: many* --- GT: DaSAC KING "The best laid schemes of mice and men go often awry, and leave us nothing but grief and pain, for promised joy!" - To a Mouse
When you jump from normal to true vault hunter mode you're going to be very overwhelmed by the mobs. I'd save the bulk of them for when you make the leap to TVHM and when you're in trouble. I'd also save some for 2.5 because everything jumps again. --- The Queen of Light took her bow, and then she turned to go. The Prince of Peace embraced the gloom, and walked the night alone.-Battle of Evermore/Zeppelin