Cutting into the waste tank? The best tool for cutting cleanly through any metal would be a plasma cutter. It would not take a very expensive plasma cutter to cut through any material used in building a truck mount.
These turbo shears would be the cats meow. I do not own this tool.
I can cut a perfect 9 inch round hole ( use a cardboard template and sharpie to trace around) in 30 seconds tops.
I use yellow handle straight cut but also nice to have these green handle snips for tight areas.
Here are the turbo shears
and 90* snips