it should have a 16 gallon tank, so if you took some gas with you and ran it for a while to determine where you can go... you should be getting 16-22mpg if I had to guess as I got 19 with the Explorer I had for a while...
If the gauge moves but isn't reliable, it's $55 plus shipping for a fuel pump and fuel sender at rockauto.com for a good delphi unit($50 for just the sender, or like $35 for just the pump... doesn't make any sense but I got the two together), with two people in about an hour you could have the sender changed. There's 6 bolts holding the bed on that take a T55 torx bit, 3 screws holding the gas filler neck in that take a 7mm socket, and a connector at the rear of the drivers side frame rail for the tail lights. After you get that undone, get one person on each side of the bed and lift it back about 2' then set it back down and now you can replace the fuel pump. To replace the pump there's two hoses going to the plate and an electrical connector, remove those then the silly plastic ring that has the notches for the hoses and the wires, now take a hammer and punch or something else that's blunt and loosen the lock ring, it is the plastic ring with ridges, turn it counter clockwise until it's loose. From there lift the old sender out, drop the new one in, and reassemble...
I just did that 2 weeks ago by myself (instead of using two people I used an engine hoist to lift the bed), while I was there I loosened the cab mount bolts and moved the cab back as far as it would go to gain some front tire clearance, with breaks and everything I think I had 4 hours in it so it's very doable in a day...