When you use gravity feed it is very important to vent the auxiliary tank in the truck tank vent. That is if you would like to see the gauge going up, and empty faster.
Let me explain. When you open the dumpvalve , you allow fuel to flow in the bottom tank, and the bottom tank needs pressure to vent out. I connected a line from the fill neck of the truck to the top of the auxiliary tank so as to vent the bottom tank back up to the top tank. Now the bottom tank air escapes to the top tank while the top tank gets its vent from the bottom tank. The bottom tank cap is now the vent for both tanks and therefore you are covered for proper ventilation. This way I just cap the top tank off, let it vent through the bottom tank cap, that is also certified. Tank certification is mostly required for pumped tanks and not for gravity units as it is just an extention of the bottom tank.
My friend had a dump tank that just vented out and he had to stop and open his truck tank cap to let the air escape in order to empty the top tank.
With a 10 micron filter and Solenoid valve, I can empty the top tank everytime, at the lower level it empties slower but does it quit well.