I got a Transfer Flow tank a couple months ago. In my research, I seem to remember that gravity feed is now severly frowned upon by DOT and regulatory agencies... . Various reasons, the main one being the potential for overflow.
My dad had gravity feed and I've had them, but the solenoid switches aren't expensive (Napa has them) and electric fuel pumps aren't costly either.
Do some searches on the Web and here at TDR and you will get a ton of info. One site has the entire DOT requirements for fuel tanks. I know it sounds like it is overcomplicating a simple matter, but we are all a lot more safety conscious than we used to be.
Transfer flow pumps directly to the fill neck of your oem tank, constantly filling up your tank as needed. A manually valve controlled line as you propose works, but it's not the safest way to go. Nor the best. And I am not sure, but I think it may be illegal.
Keep looking. You'll find the info you need.