Recommend idling RPM

The idle rpm is entirely controlled by the computer and is not adjustable at all. If you are not getting a check engine light, then the system is likely working as designed. The rpm's will vary with temperature and conditions.

If you are only using the dash tachometer the rpm is probably fine. They can be a bit off. Toss a scan tool on it and look at the live data and see what it shows. Does it have any other issues? A tiny vacuum leak could cause the idle to rise.
Normal idle on the 5.3 is around 600 rpm when warmed up. Starting cold rpm will run up to 1,000 then lower as it warms up. You might want to look at the vacuum lines and gaskets to be sure there is not a leak.

A vacuum leak will tend to raise the idle but it will also eventually also set a lean code and put the check engine light on. So again if you have no check engine light, chances are it is normal.

That is why I said it could be a leak. The other issue is what is he trouble shooting for? Very well could be related.

Well if you are like most people who put on a non-factory set of mufflers, you probably like to hear them a bit more. So you step on the gas a bit more often and maybe rev the engine a bit more than you used to. That alone can alter the fuel mileage. Now if you are driving it exactly the same, using the same fuel and everything else is the same, you should get right about the same mileage. I have seen full cat back systems get better mileage than stock and some get worse. 99% of that is due to the driver.
With the mileage you have you might think about cleaning the throttle body, and doing a top cylinder cleaning. Most of the newer engines seem to build more carbon due to the emissions systems and the throttle bores and valves bear the brunt of it. Lot's of videos on those processes out there, watch a few and see if they might help.