Wow! Over $40,000 in Maintenance... For what, besides the in-frame? To me, if it didn't include the in-frame, that's way too much, you could have sold the truck and bought a pre-2002 model, and fixed it up for less than that.
ACERT is terrible, so is any EGR system, and so is the '07 Columbia. I drove a couple of those, and even with the Detroit motor, they were bad enough with the little "maintenance" things. Do yourself a favor, get rid of the truck, go buy yourself an older model truck (perhaps something built before 2002 that already had an in-frame), take care of it, and you will be much happier.
I personally hate EGR, ACERT and DPFs (the very things that likely put you in the hole on your '07 Freightliner). This is what happens when you let the EPA, CARB and other GOVERNMENT ORGANIZATIONS shove this stuff down your throat.
There is hope... if you are willing to sell the '07 and buy yourself an older truck without EGR, ACERT or a DPF.
When was the in-frame done? 790,000 miles sounds a little too low for that particular service.