"Gravity fill alone is very slow. We added a small universal inline fuel pump to help push/pull fuel into the main tank"....
On my aux tank, I have an in tank fuel pump connected to a motorcycle battery for a power source. The aux tank is plumbed into the main tank filler tube. When low in the main tank, I turn on the aux. fuel pump while stopped at a rest area, let it pump (1 gal/minute) until we are ready to get back on the road...somehow a vacuum is established and even when the pump is switched off, the aux tank continues to fill the main, then pulls directly from the aux tank 'till empty, then draws from the main tank. I love how it works.
Regards
Gemstone