If you have a dirty water tank, a clean water tank, and some way to clean/boil the water (kitchen unit for ex, unless you get lucky and find one foodco buddy might take longer to acquire), you’ll get prompted to fill the vehicle with the clean (super simple in more recent builds).
Personally I do batches of 20 and repeat (with ') a few times so set myself up for a while. The dirty water tank with 2 funnels fills fast (and/or you can siphon pools, ponds, rivers, etc).