Sounds like you got it right, I've used brake clean and a shop wet vac with a piece of copper tube to get slime and sludge out. I was more relaxed about it after watching mythbusters vacuum up gasoline and not creating a plastic jet turbine. I still did it outside though.
I have one oil tank that seems to not have a bung in it, just a self tapping repair plug screwed into the tank it's self. That one may require a tig weld. Another may have rust, so it will get your RX for cleaning.