I recently had multiple water tanks installed, meaning I had a clean water tank and a dirty water tank in the same vehicle at the same time. The way they are separated is via different components, by which I mean you have to have a small metal tank in order to install a clean water tank. If you want a dirty water tank, you have to have another component, a 33 gallon drum, IIRC. When you install the vehicle parts, they appear as ‘water tank’ and ‘clean water tank’ in the part selection list. And if you browse on them in the selection list when they’re greyed out, they should mention which components you need. If the they don’t appear in the part list at all, the component cannot be installed in that vehicle tile, so try a different tile.