I always check my gage real good on my level driveway. Then check the trip ODO. Uphill, it looks full, and stopping makes it look like I'm empty. Or is it the other way around?
Just the nature of the beast...
Gas mileage? Remember, you're driving a Tahoe not a Yugo. I did improve mine with a K&N FIPK, dual chamber Flowmasters with 3" pipes, a little chip work, new plugs and a clean out of the O2 Sensors(85,000 miles)and about 5 extra lbs in the tires (summertime 12.5X33X15 BFG All Terrain).
Also did the Seafoam trick, and it seemed to help with throttle response. Don't put it in the gas tank, pull the hose off the PVC valve, and have someone crank it until all the stuffs gone (Won't take but 30 seconds or so). Then let it sit for half an hour, and start it up. Make sure you do this in a well ventilated area, because the smoke show is incredible. Once it's running smooth again, take it out for an "Italian Tune Up"...blow out the rest of the Seafoam and the crud it loosens up, and it really cleans the intake. Make sure you wait to do the plugs and the O2 sensors after the seafoam...
With the lift kit and all the other weight bearing goodies I've added, I'm averaging about 12 in town and 16 to 18 on the highway. Put the trailer behind it and all bets are off...9 to 10 uphill, downhill headwind, tailwind, it don't matter.