Thanks for all the help.

We took both 20 gallon gas tanks out. Filled them each with a full can of engine degreaser. Sloshed them around and let them sit on and off for 2 days. Rinse them throughly with tons of water. Let them dry 1 day and then put 1 bottle of isoproyl in each tank. Let stand another few days draining. Replaced all fuel lines and the gas/water fuel filter.

When draining the tanks, no sludge or gunk of any kind came out. Water draining from the inside looked almost drinkable. Inspection of the inside of the tanks with a small flashlight showed nothing but clean, shiney insides.

Tanks are now back in and ready to go.