I use a heat gun to expand the aluminum , then use the impact. In lieu of an impact you can hold a screwdriver and tap the end of it with a brass mallet. That shocks the screw and breaks loose the corrosion. Of course, gas is out before heat gun is used. Rarely to I have to resort to drilling, usually only when a PO has buggered the screw head.
Your phrase the gas was red bothers me, if the tank was relined whatever was used is breaking down and contaminating the gas. I'm not sure what product causes that but I have seen it several times in the past year.
Best of luck with your project.