I tried for about 3 hours yesterday. I was trying tomato-RT-AC68U-ARM--123-AIO-64K.trx. I could never get the firmware to take. It would upload and seem to complete successfully, but I could never access the admin page, even after waiting for 20 minutes to be sure that the process was complete. I tried about 6 times using the instructions here, http://www.shadowandy.net/2014/03/asus-rt-ac68u-flash-tomatousb-rt-ac68u.htm
Yes, turns out that since the board type is different for the RT-AC68P, they'll need a separate build for the RT-AC68P for tomato ARM, like Shibby has for the RT-AC68R. Not intuitive, but that's the way it goes.