There is a fuel economy gain from exhaust upgrades, but it is generally less than expected.

If you are still close to stock,
your exhaust backpressure at
2000 rpm and three quarters open throttle
will probably be
3 to 4 psi at the exhaust port.

About 2 psi of this caused by the catalytic converter, the rest by the muffler and tailpipes.

If you can get this down to 2 psi or less
you might get 0.5 to 0.75 MPG gain while towing.

10 MPG for day to day non towing fuel economy seems low.

Air your tires up to 40 psi, f
ill up the tank with 87 octane "winter blend" gasoline,
then drive 100 miles at a steady 60 mph
on a blacktop interstate without super steep hills
during a day when the wind speed is less than 7 mph.

If you do not get at least
17 MPG
there is something else wrong with the truck,
such as a poisoned oxygen sensor
or a fuel injector(s) dribbling fuel when it should be off.