This should be able to be handled with a “really big keg” container type, as suggested further up in this thread. If you create the customer’s tanks as a “Container type” instead of “Vessels”, then you should be able to create a product for them and rack into their “kegs”. These “kegs” can then be sold and delivered, like any other product.
It’s also possible to create a “part-filled product” and rack into the part-filled product. This will allow you to handle the last bit after all the fully-filled containers have been racked. This needs to be created as a separate product and when you are racking into this product, Breww will prompt you for the volume that it was filled with, rather than the usual assumption that the container has been fully filled. There more on how this works at Best way to deal with 'part fill' containers? - #5 by max
I hope this covers what you need, but please do let us know if you need more help on this 