To rebuild or not to rebuild? Keep rig or try something else?

This is kind of a 2 part question...are there any particular signs of when an engine needs to be rebuilt? Also, are there signs that you have a solid engine that will run well over 1 million miles or more?

Here's what I've got:

2007 Volvo 780 D12 465hp 748,xxx miles.

It does not use any oil between PM's, 20-22k mile interval, minor oil pan gasket seepage only. I know, all engines this old must "use" some oil, but I never have to add in between is what I'm saying.
Also, it does not use any antifreeze, no noticeable smoke comes out of the stack, and I've been looking for it when the truck is climbing/downshifting etc..

Are those good signs or not signs at all?

I am trying to decide wether to keep this truck for the long term, install an APU and do some other maintenance she needs, or to look into something else. Yes, it's a Volvo and I have put a lot of money into it, got it at 593k miles and have put about $14,000 into it-not counting the PM's or ten new tires. Worst part is it's the 10 speed autoshift. I hate the auto but glad I got the Eaton-Fuller 3 pedal one with the clutch. I have a pretty easy dedicated run so the automatic is tolerable I guess. I've done so much to this truck that I do not want to go buy another used one with 500-600k miles on it and start the process of replacing everything hanging off the engine all over again ya know? I've considered new, or a fitzgerald glider but don't want to be stuck with the $2,000 or more payment+higher insurance. Any thoughts?

 

I have not changed the diff or tranny fluids yet, that was one of the things I was considering doing very soon if I decided to keep it. Then again, I've been saying that for months, I should just do it. It uses all synthetics, speedco has reccommended it a few times, but adding $360 to my PM wasn't really an attractive or affordable idea until recently. 2013 has been relatively quiet-repair wise. Speedco has done oil analysis for me in the past, one showed high possibility antifreeze in the oil. That turned out to be correct, I was using small amounts of antifreeze at the time only to find out it was my EGR cooler letting it into the engine. That got fixed just a couple days and only 70 miles after the analysis so it could not have done too much damage. I know speedco's analysis isn't always reliable but they were right that time. Thanks for the advice. I will look into sending out a sample, and the oil filtration.

 

Hell yeah, if I can get at least another 600k miles out of this I would be plenty happy. That's about 4 more years at my current pace. I've driven those Cascadias and I hear ya. That's what got me looking at a glider rather than new if I was to change trucks. I have enough trouble here with Volvo's EGR system. Don't need DPF and DEF systems on top of it.