I think your mileage is on the low side for sure. I think the mileage you report from the Tahoe is on the high side. Is this actual calculated mileage or are you using the built in vehicle display?
I question your driving style first. You say you are shifting it yourself and not using cruise. I would think that putting the transmission in tow/haul and letting it shift itself might give you a bit better mileage. Also, I would think using cruise would improve it a bit too.
When I tow I general engage tow/haul and set the cruise. I let the vehicle figure it out.
As mentioned, make sure your tires are rated for that speed. If they are ST they probably aren't.
