Are you looking for a new truck or used? Since 2007 the emissions of diesel trucks has really increased their prices, so they are now up to $10,000 more than a gas truck with the same options.
I would rather have a gas truck because I do not go camping that many miles each year and getting 9 MPG while towing is not a big deal. Insurance always costs more per year than the gas for my motorhome. The only two exceptions to that are when I drove back east two years and put more than 6,500 miles on in one year.
The Ford 6.2L is the largest displacement truck now for towing a normal trailer. You can get the single rear wheel F-350, it is normally good for about 3,700 pounds of cargo. Don't think you need to carry 2 tons?
THe fifth wheel hitch is about 100- 150 pounds.
THe pin weight on that hitch with a 15,000 pound trailer can be between 2,500 and 3,000 pounds. You plan on carrying some cargo and passengers in the truck, so lets say you plan on having 500 pounds of passengers and stuff in the truck, 100 pound hitch, and 2,200 pounds of hitch weight. That all adds up to 2800 pounds. Should your trailer water tank be in the front compartment, and say it is 60 gallons (540 pounds) of water, that can add another 300 - 400 pounds of hitch weight to the truck.
If you are looking at used, look carefully at the GVWR. Make sure you are looking at trucks over 10,000 GVWR. SO if you are looking at 1990's trucks, you will require a dually, the single rear wheel trucks topped out at 9,900 pound GVWR until the 2005 F-350 came out and improved that number significantly.
You probably want to avoid the 2003 to 2010 Ford diesels because of the oil cooler and it's special needs. THere are lots of posts on that engine. THe 2011 seems to not have any problems, considering the million or so out there, only a fraction of them have had problems.
The 1990's GMC's had a more powerful engine than transmission. Same with the early Dodge trucks. By 2000, all three big trucks had much more powerful transmissions, and much more powerful diesel engines.
First thing to do is stop by your local Ford, Dodge and GMC dealerships and pick up a trailer towing guide. It will explain GVWR, GAWR, GCVWR, and towing needs in much more detail. Best thing to do is select a truck suitable for your needs. Don't buy a 3/4 ton because the salesman says "It will tow anything". It will not safely tow much, except for the largest bumper pull trailers (they are limited to about 1,200 pounds hitch weight, so a 3/4 ton can handle that much weight).
IF you don't mind dual rear wheels, then any brand will work fine. If you want to stick to the single rear wheels, and not have those extra wide fenders, then consider a Ford, it has the highest GVWR of all the F-350's with single rear wheels.
Fred.