<<How do you get the tank to fill rapidly, even with a rapid flow rate?  I
have to keep the flow rate very low, or water fills the filler tube and
overflows.  It takes a long while for me to fill the tank, being careful
to avoid overflows due to backup as I go.>>

That was going to be my question as well! A person would think that I have a 100 gallon tank inside the van with the amount of time it takes to fill that small 13 gallon tank. I've always had to keep the flow down to a slow trickle, although at times I thought about removing the screen to speed the process.  Since the tube doubles as a vent, the water would probably come gushing back if filled too rapidly. The solution would be to add a vent, but it would have to terminate higher than the fill valve to avoid a leak, at least that's my theroy;)

Troy