People claim all sorts of MPG numbers. Many drivers assume the modern in dash MPG meter is accurate. Some are but many are not. Unless folks actually write the numbers done, and get a good average of at least several tanks, and preferably over an entire multi-thousand mile trip, then the numbers are likely misleading.
If your Tundra is one with the 4.30 gears, six speed gearbox and 5.7 engine, then yes those are geared in a very effective and efficient way for towing. Having low ( high numerically ) rear gears mated to wisely chosen trans ratios is the best way to gear a drive train for pulling heavy loads. That engine is tuned for a lot of power, so fuel mileage tends to suffer somewhat to get that performance.
I suspect your K&N filter neither helped nor hurt your mileage to any real measurable degree. I'm not a fan of that kind of filter, but it's not related to any performance or mileage issues.
Out of my little 4.0L Frontier with manual trans, pulling 4000 pound Funfinder, I get 8 to 11, with my average coming in right at 10 for most trips. I tow at around 58 to 60 mph at about 2900 rpm.
Our Ford diesel gets 8 to 11 pulling 15K pound horse trailer at somewhat higher speeds.