As a commercial real estate agent I have sold the occasional business. The IRS is very well aware of the typical cash flows of various business, but usually doesn’t go after them unless they start posting crazy ratios. Not reporting all cash transactions is pretty much par for the course with the majority of cash handling businesses. It’s not that difficult to know what a business is really doing if you’re aware of the sales ratios. As an example experienced convenience store operators can simply look at the milk purchases of a convenience store and nail the real total sales to within a few percent. This is true for a vast variety of businesses.
The dump truck schemes sound more like internal tax or budget games of some kind not true money laundering. The buying and selling transaction of the trucks will leave a hard paper trail.
It seems to me a business involved in gambling such as a bookmaker or a casino fits the criteria. They can just claim to have got lucky, thus making a much bigger profit than their competitors. It would also be easy for them to falsify records as most of the transactions are in cash.