I have a 2004 Minnie and have it shut off pumping too early. I am able to get another 5 gallons of gas into the tank after the first clink off. What I have done it use my scan gauge and see how many gallons of fuel I have actually used and refill it to that amount. It's a few gallon shy of actually being topped off.
I can't tell if yours is on a Ford or Chevy chassis, but the Scan gauge has a tranny temperature function. Well worth the investment. Hooks up to the OBD and reads the computer. Under TANK function, you'll get the MGP, miles driven, miles left in tank, gallons used and gallons left.
Here's the link
Scan gauge