Try this, pull the drain plug on the transmission and dump the fluid. See how much comes out; it should be about 4 gallons. If there's a lot more than 4 gallons, that might be your problem.
Next, if you can drain the transmission and get the plug back in before the air leaks down great, if not, you can hook up a compressor to the emergency glad hand on the front and pressurize the air system and see if air still comes out of the transmission. Do NOT start the engine with no fluid in the transmission.
Put the fluid back in it. If you want to get new fluid, this is what I used. It has the Allison C-4 spec.
Follow the fill directions in the TM. I don't remember exactly, but I believe you fill it until it just shows on the dip stick. Then you have to warm it up and shift through the gears (read the TM) and check the fluid level when the transmission is at operating temperature. Some dip sticks have a cold fill line, mine did not. If you fill it to the add oil mark when it's cold, it will be over full when it's at operating temperature.
Does it puke out fluid if there is no air pressure? Open the emergency glad hand so the truck doesn't build air pressure and see what happens.