Carl, That will work if his cap isn't venting, to an extent, but doesn't address if the tank is too full!
Matt, Several other folks above have mentioned that "topping off" in-cab mounted tanks to the point it leaves little to no room for expansion will result in the gas overflow problem you described. The fill tubes attached to these tanks leave very little room for expansion to begin with and yes, gas does expand A LOT when it's temp increases between in-ground cooler storage tanks to sitting in a hot sun enclosed cab and tank.
I will admit to making the very same mistake when I first got my Grandpa's truck. Filled it up, then gave the handle an extra squeeze after it self-shut-off like I have always done on all my other vehicles. An hour later sitting in the hot sun and I had gas dribbling down the side of the truck cab, leaking through my properly vented gas cap.
Best rule of thumb when filling these tanks is at a minimum, don't give that handle an extra squeeze when it auto shuts off. Better yet, if your truck is going to be parked anywhere in direct sun or inside a hot garage, etc., leave it a little short when filling. These efforts will help you from a repeat overflow performance.
