I am actually more sympathetic to original poster's plight.
Not that there can be any way of proving that the room steward (or anyone else for that matter) took the Ipad. You can't.
Yes, the OP was responsible for accidentally leaving it behind. But....
I've left a few more minor things in cabins along the way; a windbreaker hanging in the closet and the like. The cruise line's "lost and found departments" are pretty much useless. When you contact them, they say the will "investigate", but it's rare to ever hear back from them.
In this case, it sounds as if they didn't even respond enough to take the call.
In my view it is much more likely that an RCI staff member (whether a cabin steward or someone helping to strip the room), than a passing guest found the Ipad on the bed, realized the guests were gone, and took it.
If found by a cabin attendant it could have easily been turned in to guest relations, have them check the guests name, and have head office contact them via email or through their emergency contact #.
Sadly, I don't think any of them go that far.
However, the bottom line is RCI is not responsible for the loss, anymore than some beach bar would be if you left a camera on the beach.
It is quite possible that the poster's home owner's insurance would cover the loss. Many of these policies do cover stuff like that, not just items within the home.