I have in recent years had:

99 Dodge Cummins dually
03 Ford F350 6.0 SRW CC
06 Ford F550 6.0 CC Dually
04 Chevy Duramax CC Dually
99 Chevy Tahoe (two of these actually)

By far the BEST milage has been the Chevy duramax. Loaded with two cars and equipment in a STEEL 40'f goose I averaged about 12.5mpg. Around town unloaded I got closer to 20-21, depending on how much highway I drove etc. Light or unloaded on the highway I would easily see 23-24mpg. The Dodge was OK, but it is an expensive bastard to maintain, and while the powertrain (5speed, not auto) was almost bulletproof, the rest of the truck is a POS. The two fords had good interiors, but the 350 rode like crap unless you had a very large load behind it, and the 550 never got better than 15mpg ever, unloaded, and about the same loaded.
The 99 Tahoes got 18 or so around town with me driving them, 16 when my wife drove, 9 to 11 when towing a 26ft enclosed trailer and one race car. However they were the easiest to park
The Dodge and Fords all had fairly consistent steering issues. Solid front end/kingpin arrangements mean ball joint wear and the need to be rebuilt/adjusted/etc. fairly often. The Chevys do not suffer this issue. With 280k on it the Duramax literally steers/rides like a brand new truck.

I would not recommend a diesel for an everyday driver, the cost involved is just not worth it. I would stick with a solid 1500 truck with a decent size engine- no V6- and suffer the milage penalty knowing that if you need to, you can drag a boat, camper, etc. behind you. The 99 Tahoe 4x4 was rated for 10,000lbs. The Cayenne (surprisingly) is rated for 7000lbs. Iffin it were me, I would purchase a single cab pickup if it was me and a hunting buddy, or a Tahoe/Explorer/etc. medium SUV and a small trailer if I needed to haul around more people on a regualr basis. Parking CC trucks- especially real trucks with a real bed- is not as fun as you might think it is.

As a sidenote my buddy just purchased a new Chevy extended cab fullsized truck and I think he is getting 22 or so mpg on the highway with the cylinder drop system, and another freind has the Dodge 1500 with the same sort of system and gets about the same milage, but they can both tow a reasonable amount.