Drove 500 miles in the last 2 months and the system has lost 2 full quarts of ATF.

Any thoughts on where to check ? Where the truck is parked is rough gravel and it's not leaving me a tell tale sign.

 

There should be a pretty good trail out of the leaking component. Clean things off, run it and look for the leak. Time consuming, for sure. You may want to try a big piece of cardboard under the truck when you park it for an hour or so, that might give you a pointer. We have Cummins/Allison/hydroboosted trucks and they all leak and drool.

 

Is it leaking into the cab?

 

No, its looks to be coming out the fill cap and vent tube