"Chips" is the term many people use to refer to performance adding electronics to a vehicle. Chips is technically the incorrect term to use for these trucks, since the computer can be reprogrammed or fooled with add-on electronic boxes, but that is beside the point for now. Chips are also a tasty snack, although they aren't all that good for you unless you get the baked ones. (Sorry, it had to be said)...
The question of whether you want to add a "chip" to the truck can be answered only by you (i. e. do you want to add power, void the warranty, etc). "Chips" are risky, although the extra power can be fun, and in many cases can help fuel economy a little bit if you can avoid using the extra power.
13-14 isn't all that bad in town. I'm not sure how you drive, but if you feather the pedal you'll get better mileage than if you want to put it down and get up to speed in a hurry. Does your Brother in Law (hereafter referred to as BIL) drive in the same town, same time of day, and so forth? Do you both take the same routes with your trailers in similar conditions? If not, then it'll be more difficult to compare. How fast do you drive out in the country and on the expressway? If you're exceeding 70, you're hurting your mileage. Accelerating up hills can help kill mileage as well, instead of just easing up the hill and regaining your speed on the down side.
Are the tires inflated to the correct pressures, all recommended maintenance performed and so forth? How about the filters (air, fuel, etc), have those been changed recently? What size tires do you both have? What gearing, and automatic or manual transmissions? These things can all make a difference.
As to the differences between the trailers, part of that can be explained away by aerodynamics. His fifth wheel sits closer to the cab than your bumper pull travel trailer, thereby helping to reduce drag between the cab and trailer. The more turbulence you can find a way to reduce, the better your efficiency will be. How fast do both of you drive when you're pulling trailers?
Finally are you sure you and your BIL are taking mileage readings in the same or similar manner? I. E. both filling up until fuel is visible in the filler neck, and so forth? SOme of the differences could be attributable to that. Don't go by what the overhead computer says. It could be off for a number of reasons, including "chips".
I can see a several MPG swing in my truck's mileage if I drive it reasonably or if I'm hammering it, etc...
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