I just want to add a some words of caution. I have a friend who owns several car washes and he has a couple of folk who service his vacuums, He has them dump the drums once a week because you never know what the customer sucks up, from dust, to dirt, to mud, to vomit, to poop, to (every other objectionable, obnoxious, disgusting, detestable, nauseating, bit of debris imaginable and a few you cannot imagine…).
If you are going to use a car wash vacuum, try to find out if and when it was cleaned. Each Vac has a dust filter as well as a dump drum. I have accompanied him on several occasions when he receives reports of broken equipment or vandalism and he changes out the dust filters about once a month. He bought another car wash that was for sale about a year ago and I went with him to inspect it after closing the sale. The Vacs all worked, but hardly sucked. The dumps were full and the dust filters looked like that had never been changed.
I use a small (one gallon) Craftsman Wet/Dry Vac and I do it often enough that the little Vac is all I need.
Oh, and one more point, my friend puts wire ties on the Vacuum’s access doors in an attempt to keep the kids and bums from dumping the debris out looking for loose change (coins that are vacuumed up) and they do not always close the doors. If the door is not sealed (left open, or ajar), the Vac will draw most of its air through the door and not the nozzle…