I've found that the last couple years of the 7.3L are the only diesels to be trusted if you need a Ford. I'm a Ford guy, I've owned 5 Ford trucks, and currently own 2. I've been driving my '00 1 ton since spring 2004, put over 100K on it since I bought it.
Originally, I was getting 12-13mpg consistently going 65-70mph on the highway, unloaded. It was my daily driver, about 2 hours each day for a couple years. When diesel hit $5+ per gallon in Alaska (2006 I think? Don't remember), I started looking into MPG mods. I did a lot of research, and ended up installing a Edge Juice computer chip with Attitude in-dash controller & a Banks transcommand. I threw a K&N filter in the stock box and upgrade the exhaust from the turbo back to free-flowing all stainless MBRP, 3" to 5" at the tail. It took some messing around with the various settings to find the sweet spots, but my mileage jumped to a consistent 25mpg @65mph unloaded (almost no hills on the commute). I found that if I slowed down to 50mph, on a long trip and didn't make any stops...I could squeek that mileage up to 30mpg. I did that coming down the alcan unloaded once (carried nothing but fuel tanks, and came down solo with the dog and a sleeping bag did it in 47 hours). I had a boat back then that weighed about 8K on the trailer loaded for fishing/hunting, I could get 18-19mpg towing it to Seward. Then I broke a wire harness in my valve cover. Too high of oil pressure, starting sprouting leaks.
I de-tuned the juice chip to mid level (It was maxed out), I lost 2-3mpg across the board, but haven't had a problem since. ULSD changed mileage a bit, but once I discovered additives (Howes preferable) I got that back. Tire choice makes a 1-2mpg difference also. I got better mileage when I was running highway tires with low rolling resistance.
I've got about 120K on it now, still running the same mods (performance wise at least), got a bit more aggressive tires now, just came up the Alcan in late September ~5K on a trailer with the AF1150 loaded heavy and the cab full, averaged 17mpg over the whole trip with an average moving speed of 40mph. The Uhaul following me slowed us in the hills, but not much. The conditions of the road force you to slow down considerably. My trailer was overweight, and had to be nursed up the Alcan.
I've never seen the point to a 3/4 ton that would be used for towing. But as a comfy low weight hauler (dirt bikes or something), I think if you outfitted a 3/4 ton with 3.73 gears and taller tires, low rolling resistance, did some mild tuning with free-flowing exhaust...you could probably get a 7.3L to consistently turn in 28-30mpg numbers unloaded. I'd be comfortable buying a 7.3l at 100-150K miles if the maintenance is documented well. Regular oil changes with a quality oil used, axles maintained, etc. My transmission is getting close to rebuild, I've got at least 60K miles of towing, and 80K miles of pushing the HP to it. I've still got the original transmission and torque converter, and they're tired.
As far as maintenance, oil changes are expensive. I use a 5 gallon bucket, and drop nearly $100 doing it myself. It holds a lot of oil, 3.5 gallons roughly. I broke an axle once, that could have been prevented. Cost me about #3K, partly because I was stuck in the middle of nowhere Arkansas over a holiday. I had a water pump changed under warranty right after I bought it, I had to replace my primary alternator around the same time. I changed one CPS sensor, but after the new one haven't had to mess with it since. I changed one glow plug relay, but I think my glow plugs themselves are short on life now. Then there was the valve cover gasket (which is also the wire harness for that side), I think that was an issue with chips turned up too high, and not giving adequate cold weather warm-up time (it was -20 outside when it happened and the truck had been cold soaked, not plugged in). That's it though, pretty good maintenance history for ~120K and nearly 10 years.
My BMW 540 cost me that much in 3 years easily. But it was a lot of fun to drive! :) I also sold it after 3 years for $5 less than I paid for it, so I did ok on that deal.
I can't speak to other makes. In the military, I saw a lot of trucks get abused, rode hard and put away wet. The 7.3L Fords held up well, the 6.0 Fords did not, the 6.4 Fords had the cabs fall apart (chassis got totaled before engine). The dodges 5.9 did ok, but for whatever reason weren't popular. The duramaxes chassis fell apart very quickly, and the engines weren't doing well either. That's over a time span of 11 years, 2002-2013 that I was part of the management of a large fleet of government trucks used for flight line support of aircraft in Alaska.
When I started the Ford F350's with 351W were the favorite, many had nearly 200K on them, all hard miles towing around power carts all day, running 14-18 hours a day, constant stops and starts. Hard miles. The newer gas trucks couldn't keep up, and most eventually became 7.3L Fords, which held up well and eventually got transferred out and used again elsewhere. Nothing new has held up well since the 7.3L's were transitioned out of the fleet in ~2006. Most trucks make it 2-3 years at most and are trashed.
What I learned....was buy a 351W for a daily driver (I ended up with a '96 Bronco EB), and a 7.3L for a hauler. Still happy with that decision. 200K on the Bronco and 120K+ (I forget the exact mileage on it now, it's been in the body shop for 2+ months after a high school driver smashed it before thanksgiving) on the F350. Hope to still be driving both in 10 years from now. Especially since the F350 has a new paint job :)
