Welcome to the forums!
At the bottom of every brochure and on the website they have this little disclaimer..."Specifications subject to change without notice"
I agree that there is a pretty large difference between 22 and 39.
One trick you can use is to get a twist on valve
that fits over the end of your dump line, then leave the gray tank valves open, this will allow the tanks to equalize. When you go to dump, you close the gray valves first, then open the outside valve, to dump the water in the pipe, then proceed as normal.