Big thing with towing is where, what, and how you are towing. Lets say you are towing your trailer up and down hills in a mountian area the diesel is going to win every day. Towing around town, short hops, lower speeds (below 50 mph), you may not see such a big differance.
Case in point. I stated earlier my neighbor has a 2007 Classic, non dpf 3500 SRW 4x4 Dmax 3.73 gears. It gets about 20 mpg on the highway at 70. Any faster and it drops considerably. Around town it averages 12 mpg for him. He is a commercial mower and tows a tandem 16' landscape trailer 4 days a week pulling 2 ZTR's at 1,200# each plus the trailer at 1,500# for a total of 3,900#. Now he just got back from a trip that was 9,000 miles over 4 weeks and averaged just over 11 mpg pulling 12,000# 5th Wheel.
His previous truck was a 2005 F-150 5.4L 4x4 with 3.73 gears. He pulled the same trailer for his landscape business. He got 13 mpg average, actually better than with the diesel. It would only do 16 on the highway though with no load compared to the 20 mpg his Dmax gets. He pulled a 7,000# travel trailer back then and got 10 mpg versus the 11 mpg the diesel gets. Of course the diesel is pulling nearly twice the load and does it much better but there is no significant increase in mpg while towing.
As for maintenance I do it all for him except for the warranty work. I do preventive stuff on all his equipment including dump trucks, tractors, dozer, mowers, trucks, suv's, go carts for the kids, ect. Sunday I did a 60,000 mile service. I put on a new fuel filter @$40, new air filter @$65, a oil change 10qts syn 5W40 and filter was $75, and rotated the tires which are nearly worn out (2nd set). As you can see its expensive. We do 7,500 mile oil changed and all other filters are at 20,000 miles.
Chris