I had a 7.3 liter Powerstroke 1-ton that went 140k with heavy towing and the only thing I really had to do was fluid changes, tires, and a couple sets of wiper blades. I only sold it because I wanted a 6spd manual transmission so I could choose the gear, rather than having to live with what the truck wanted as with my 4spd auto in the Ford. While on-the-road, I'd often compare notes with commercial haulers, and frankly I ran into MANY 7.3 liter F-350s with a half million miles with ZERO major repairs, and they were hauling 12k+ pound trailers day-in and day-out.
My current '03 Cummins only has 139k on it, but it's been very reliable as well. The fuel economy is almost unbelievable (40+% MORE than the new 6.7 liter Cummins produce, and 25% better than my Powerstroke). What's not to like? Most of the "Hot-Shot" operators here in the Texas oil fields favor the 5.9 liter CRD and have almost no problems with them.
I cannot speak for the Duramax (except rumors from unhappy haulers at fuel islands), but I can tell you I would not hesitate to buy a clean older 7.3 Powerstroke, or a 2003-2007 5.9 liter Cummins powered truck for a small fraction of the number you quoted for a new truck. Frankly, with Cats, EGR, DEF and the emissions systems found on the newer diesels, I wouldn't choose to buy one, and PREFER the older, more reliable engines I mention.
I LOVE the versatility of a truck camper, which allows me to use my truck for other purposes when the camper isn't on the back (I have a 5th wheel beside the T/C, and haul cargo trailers at other times).
I am sure with the service records you kept, there is a buyer out there who would probably pay a premium to get a well-maintained (too well-maintained $$ wise?) truck, leaving you the possibility (probability) of finding a low-mileage used truck that will be less of a financial burden.