James, what are these vac trucks used for? Are these oilfield trucks? Being an ol' roughneck, I have seen hundreds of these over the last 25+ years. There is no way I would be cleaning those nasty dirty trucks without recovery. If I were you, I would purchase the wash pit, recover the waste-water, and ask if you could use one of their trailers for disposal tank. It's a win / win situation for both you and the company.