There is nothing different to any other PO, you create a PO in ME21N and do a best guess for the quantity. Keep an eye on the over and underdelivery tolerance, as it should be wide enough.
And finally you receive what you get.
There is not even a need to for an alternative unit, as it does not really matter if you order 1 tank truck or just 23000 kg (or whatever the average size of a tank truck in your country is)