If its really bad here's what I do. I put a few handfuls of 2B limestone gravel in the tank along with about a quart of acetone. Get you and a buddy to slosh the snot out of it. The acetone dissolves the gunk and the stone gets the stubborn stuff. Don't worry, the stone is big stuff, like the size of the end of your thumb and its not "dirty" -- as in having lots of stone dust and little bits along with it. Its just big stuff so its easy to get out and easy to tell if you've missed one.
I've also had pretty good luck sticking the nozzle of a 3500 psi pressure washer in the filler neck, but since you can't be assured of hitting all the gunky stuff it never inspired much confidence.