I don't have a DeeZee, but my "Transfer fuel," 50 gal. gravity feed, Aux tank, will refill my 30 some gal. truck tank in a couple hours while I am driving.
I Do have a vent on my aux tank, Don't know if it's needed.
Like others have said,The truck computer doesn't understand why the truck is moving, but the truck fuel gauge is not emptying, so the fuel gauge defaults to empty. I ignore it because I know I have at least 50 gal. more fuel. The gauge will reset the next time I turn the ignition switch.