Fuel tank draining and cleaning

I not-so recently purchased an 82 V20 with OMC 5.7 i/o. The boat has sat for at least 10 years with a full fuel tank.

I have drained most of the fuel out but there is still some residual fuel left. I would like to completely drain the tank and clean it with out removing it. The tank has no leaks and it appears to be in good shape. What methods can be used to fully drain and clean the tank while still installed?

I have installed a new engine and had the outdrive rebuilt. I can't wait to get it out on the water. There are just a few things left to do. The fuel is the major concern at this point.

Thanks in advance,

BassOMaticII