W211 BEAST

This has nothing to do with E55s, but you guys are the most knowladgeable on the net.

A friend of mine has a 1987 BMW L6 (635CSI), it sat for a few years and then he drove it for about a year and now he wants to clean out the gas tank. The only option he knows about is to take the tank out and clean it out. He also told me that there's a drain plug for the gas tank. He was wondering if there was a way to clean it while it was in the car. Is there a cheaper/easier way to do this other than taking the tank out?

Thanks guys.