Taking the plunge. My journey as an O/O.

Your fuel economy should slowly improve as things break in, but that's horrific. I'm trying to figure out why it was so bad before all this. If your mpg is 6.8 with that truck, that is the very first thing you should be focused on. Something is wrong.

 

Forget about the manual vs. auto. Manual is just as good if not better depending on driver. It's true.
I see your point with high idle time though. That truck hauling dry van should be in the low to mid 8s unless your on a dedicated bottled water account in the Rockies.
Idk. Just something to consider as if you don't have enough. Lol.
An aero truck with a DD15 or series 60 for that matter should not be in the 6s hauling dry van freight.
Not criticism, but I would be looking for improvement there considering the amount of miles you turn.

 

Well, the best thing you have going for you now, is the fact that Freightliner did the work, and they’re everywhere, if there’s a problem.You probably have a loose hose clamp

 

I wound not have been too surprised if there were a few little hiccups like that after being so far apart. If the cylinders are all new it will take some time for those to break in also. I don’t think your MPG is that far out of line if you are on an account that loads heavy and are including the idle time. 7 if lucky maybe.

 

I find external coolant leaks easier to just diagnose and fix myself. You can typically do it in less time than it takes for them to even check the unit in and get it into a bay. You can also feel better knowing that you fixed it, rather than having to take it back again or end up doing it yourself anyway if they didn't correctly repair it.

There are usually only a few coolant lines in hard to reach places. First I'd give the hose clamps a quarter turn to see if it fixes it, and another turn if it doesn't. Then I'd replace the hose and clamp, or cut the hose if you have excess and replace the clamp. If there's still a leak, I would replace the fitting. You can get coolant pretty much anywhere, most hoses from auto supply shops, and specialized hoses and fittings from the dealership. Make sure your coolant filter is tight. Check the shutoff valve and housing before the filter if you have one.

I've tried plastic welding and using plastic epoxies for the expansion tank with limited success, but only as temporary fixes.