Cummins ISX coolant loss

Ok guys, first post and it's a challenge- I've gotten some pretty good info off this site before but this one is a bit odd.
In the fleet here, there is a 2008 Peterbilt 388 with a Cummins ISX that is losing coolant at about a gallon a day (around 350miles). There is not an external leak anymore, it previously had a leak at the coolant filter, filter valve o-rings, and then the fill cap on the surge tank. Now it's losing more coolant. There isn't any white smoke. The EGR cooler is not original, but I don't have any info on when that was changed.
So far I have:
Changed coolant filter, changed surge tank (old one was cracked, replaced with Dorman), coolant filter valve o-rings, new surge tank fill cap (genuine Paccar), put a hose from the pressure cap nipple to the frame so i can see any evidence of water loss- None so far. Today I added a catch bottle so if there is stuff coming out and drying up maybe I'll see it now. There is more coolant on the secondary side of the surge tank than on the other trucks, I don't know if that indicates a defect or is just a design thing.
I did pull the EGR venturi tube off the intake tube, and found liquid in it. The tubes were all fairly clean, just a light coat of soot, but the liquid was odd. I dumped it and then rinsed the venturi tube out with water into a bowl. There was no sheen on the water in the bowl, just the soot swirling around in it, so it may be possible that this is my missing coolant, but is it possible to ingest that much coolant and not have any smoke?
This truck has been an ongoing headache for me, I only started here three months ago but this truck has spent more time in the shop than the others by far. It is fittingly numbered 13, I just refer to it as my unlucky 13.

 

Thanks, the A team van of knowledge strikes again-
I was figuring that might be the issue, and what I have read about these blocks "settling" makes sense as far as the timing of this failure. The guy who was driving this truck for a short time was extremely abusive to the trucks, he blew multiple sets of motor mounts bolts out of another Kenworth here, and I guess when you run a truck harder it can cause the block to move around and break the cooler internally.
It does make me wonder if the external leaks were a symptom of the issue all along, if the higher pressure of the exhaust gasses was pushing beyond what the pressure cap could vent and was just finding all the weak points.

These emissions systems have a pretty steep learning curve to them, I finished school when the 3406E was the latest and greatest and EGR on big engines was just a campfire story to scare the young mechanics. I had about six months on early C15s, but this is the first place I've had to deal with 2007+ engines. Can't say I'm impressed, but maybe I should thank the EPA for the job security?