Fuel Gage location in the gas tank

Just wondering... since I've leveled out my truck, it seems that the gas light comes on sooner than before it was leveled out. Just last week the light came on and when I filled up I was only able to add 17.9 gallons to top off the tank.

I was wondering several things:
1. What is the capacity of our gas tanks?
2. Could it be possible that the fuel gage 'meter' is in the front of the gas tank, and when we raise the front of the truck we 'push' the gas to the back of the tank getting a reading telling us that we're running out of gas before we actually are?

Thoughts?

 

Our tanks are listed to hold 21.0 gal. I have heard when the light goes on, there's still about 3 gallons in "reserve". I have been testing how far I can go with the light on, and I have gone as far as 58 miles prior to filling up. Not sure where the gauge is located in the tank? Probably not many who have had to replace it... but maybe some will let us know...