I level faster outdoors than in dungeons because I get tired when doing dungeons and need breaks all the time but outdoors I can do a level almost nonstop so even if the xp per hour is lower it is higher for me because of less breaks.
Before lvl 30 warlocks have really low aoe damage also (22 dps at lvl 29 with rain of fire, 83 dps at lvl 30 with hellfire).
Even with hellfire I am also struggling to get 18k xp per hour in RFK (in reality I probably get ~6k xp per hour because of all the breaks I take).

I currently don't know many AoE spots but want to know more since I run a aoe team now.

For single target dps with shamans I have grinded harpies in northwestern barrens (~12k xp per hour with 10 shamans), the mobs in stonetalon mountain windshear crag mine worked well also (14k xp per hour with 10 shamans), Murlocs in hillsbrad (17.5k xp per hour).

Ofcourse my numbers are really low compared to Hardcore Scape who says he got 42-55k xp per hour when aoe farming SM.