I've added a few shelves and a drawer so far. Ideally you can figure out where the studs are, but so far I've had no issue using 1 1/4" sheet rock screws into the panels of the ward drobe closet (ie, into an exterior wall) and bathroom (interior wall). And for the bathroom shelf, I used sheet rock anchors for a little extra strength.
Here's my list of mods, including the shelves:
My 2013 Roo 23SS mods
Another member found what I think are really good brackets you can use, if you want to go the solid shelf route:
Roo Mod / Wardrobe into Pantry