Here’s ‘The Bottom Line’ (I.M.O.):
1. If 'The Math' with respect to 'How Much Truck' you need 'says' you 'NEED' a Diesel, buy a Diesel. (Study the truck manufacturer's towing guidelines).
2. If (1) (above) is NOT TRUE: If 'The Math' says that you don't NEED a Diesel but 'you're close' (say running upwards of 85%-90% or more of the truck's tow capacity *AND* you will be towing 'a fair bit to a lot of (the) time' in hilly/mountainous terrain, then you might *consider* upgrading to/buying a diesel.
3. If you plan on *towing* A *LOT* (*many* 10's of thousands of miles AT LEAST), you might consider buying a diesel even if you don't actually/technically NEED one.
4. If you just simply WANT a diesel/price is no object, then buy a diesel.
5. OTHERWISE, buy a Gaser.
Yes, *in comparison with* gasers that are towing trailers within the truck manufacturer's towing guidelines (i.e 'apples to apples'), diesels WILL get better MPG and the towing *experience* will be nicer. ***BUT*** it generally is REALLY HARD to recoup the additional initial purchase cost and maintenance costs in actual fuel savings. (And 'Gawd Help You' should you need repairs with a D.).
I spent an ENORMOUS amount of time trying to decide how much truck to buy back in '08 when I picked up new Cougar 291RLS 5er (advertised at 8125 lbs dry, with a normal tow weight probably in the 9000 to 9500 lb range (I always tow/travel with full water).
Did SERIOUS math/research.
Math showed I would have to ACTUALLY *TOW* BETWEEN 70,000 and 200,000 MILES to recoup the cost of the D. on fuel savings.
You can buy an AWFUL LOT of gas for the $8,000 to $10,000 premium that a (new!) diesel would cost.
For example: just to get a VERY ROUGH, 'BACK OF THE ENVELOPE', 'BALLPARK' FEEL for this:
Assume that both diesel and gasoline cost $4.00 per gallon. (CURRENTLY (I checked), gas looks like it is between 5% and 10% less BUT 'lets assume'):
$10,000 will buy 2,500 gallons of *fuel*.
At 10 MPG,(either fuel), the cost per mile will be $0.40 (40 cents) for fuel.
At 13 MPG (e.g. diesel), the cost per mile would be $0.40 * 10/13 ~= 31 cents per mile. (Let's say 30 cents per mile, to be generous IN FAVOR OF the diesel).
That's a saving of 10 cents per mile.
BUT: you paid (assumption) $10,000.00 MORE for the Diesel SO (*in this example*) it would take *ROUGHLY* 100,000 miles of TOWING to get your $.$$ back. (The actual cost per mile was about $30.8 for a savings of 9.2 cents per mile AND, as mentioned, gas is currently 5% - 10% less 'on average' (I checked) which would make it take even LONGER to 'get ones money' back.
Modern gasers, ESPECIALLY with DFI (Direct Fuel Injection, which tends to bring diesels TYPICAL fuel economy advantage down, especially when running lightly loaded, to around 15%) get pretty darn good fuel economy (relatively speaking). Diesels tend to WEIGH MORE and 'stop and go driving' tends to eat into their fuel economy advantage also. And, as mentioned, there the maintenance and (possible) repair side to worry about. (Oh. and diesel don't tend to make as nice a 'Daily Driver' as a (generally lighter; say 1/2 ton gasser (if that's a possibility), especially in colder winter weather, and DON'T 'make Heat' as much/fast)
One can 'drive oneself nuts' agonizing over this.
Personally, I wound up with an '05 GMC Sierra 4x4 Crew Cab 4:10 gears - tow capacity 9,900 lbs - and I don't regret my decision.
In the end, I followed (sssshhhh! Consumer Report's recommendation): "All the vehicles tested" (in '07-08 somewhere - 1/2 ton Gasser's and 3/4 ton diesels) "will tow their Maximum Rated Loads *ADEQUATELY*.
If 'Adequately' ain't good enough for you, then 'upgrade'.
I don't drive 'FAST'. I generally tow around 55 MPH '4-lane' (ya gotta be able to STOP these things once in awhile, and then ST tires aren't really rated for high(er) speeds), 'drive the speed limit' 2-lane) and on mostly flat/slightly rolling terrain, I AVERAGE about 9.75 MPG (U.S.) and I HAVE on occasion gotten as high as 11.25 MPG.
Hope this helps.
