Do you have to drop the back axle get gas tank out of an 1988 Chevy Camaro RS?

No, you generally don't have to drop the rear axle to remove the gas tank from a 1988 Chevy Camaro RS. The tank is usually accessible from underneath the car. However, you will likely need to disconnect various fuel lines, the fuel pump wiring, and possibly the tank straps. It's a job that requires some patience and mechanical aptitude, but it's typically doable without resorting to axle removal.