If you can't deal with the tank filter to drain it. I"m gonna suggest that you can disconnect both the supply and return fuel lines before the filters, and use compressed air in either the return or supply line, and put a hose on the other line and run it to a catch can. 5 gallons at a time? and with low pressure, maybe 5 lbs of air, pump the fuel/gas mix out of the tank. You may not get it completely empty, but you'll get enough so you can fill the tank with diesel and put this misdeed behind you.-(edit add) On another note, this same thing happened to my wifes brother. His wife confided many years later it was a 5 bj mistake. good luck.