I have put 19.2 at one time. I didn't mean for it to go that low. The next two service plazas that I wanted to fill up were closed so I had to go so far that I nearly ran out! I learned my lesson but without any failures. The tank is listed at 18.5 gallons but there is a two gallon reserve, which you should really never dip into.
I discovered I scream the same way whether I'm about to be eaten by a great white shark or if a piece of seaweed touches my foot.