I have no idea what the total capacity is either, but I have heard not to put any weight on the rear slide while it is retracted in. The older units would pull the supports out of the ceiling with any weight on them while bouncing down the road. Not sure about the new ones, but just as a precaution I don't ever load it while it's in. Gaffer222 has been info than me though if he's talked to a factory rep.
I'm sure at a minimum it's rated for 800 lbs.
On a side note, I absolutely love my Outback Terrain. What model are you considering?