Another way to do it is to examine the vehicle, move the cursor over the appropriate tank, and press ‘f’ to fill - this will dump ALL of that type of liquid from you to the vehicle (so for ex, if you had 10 bottles of water on you this would quickly put them in your clean water tank).
Some other tips for this are, if you have a rubber hose and park near a water source like a pond, pool, river you can “siphen” the source to fully fill your dirty water tank(s).
Sometimes it takes a while to get a FOODCO kitchen buddy in your ride (which does wonders for purifying large quantities of water), so don’t forget about batch crafting…you can do up 20 water at a time to fill your vehicle if you have a kitchen unit (or other means but kitchen units are usually easy to find). Shift +7, “f” to find “clean” water, “b” for batch, hit “y” when prompted to fill vehicle and hit any direction if you’re in the vehicle.
Dunno if it’s from experimental or the vehicle addon pack but having a couple of funnels on your vehicle does wonders for helping keep your dirty water tank topped off.