Mine is piped into the sending unit cover, thru a bulkhead fitting. Swagelok stainless fittings, all 1/2", cause they were free, lol.
Anyway, I used a push lock adaptor with about a foot of hose between the oem tank and the extra tank, for flexibility.
On the bed mounted tank, once again, 1/2" ss fittings, a standard ball vlv, and a 3/4" solenoid vlv. I stepped the solenoid vlv up in size to keep the flow rate a little higher, and it was only about 10$ more. I tried a 1/2" vlv, just took too long to gravity feed when the extra tank got low i.e. very little head pressure.
To run the solenoid vlv, I mounted a lighted switch in the dash, and I just watch the oem gauge, when it gets to the full mark, turn it off. It takes about 25 minutes to fill from a qtr tank to full, when the auxiliary tank is full, and about 35 minutes when the auxiliary tank is about empty. So, not fast by any means, but when you've got 1200 miles to go, who cares, lol.
I pulled power for the solenoid from my fuel pump harness, however, I have a dual fuel pump setup, with relays, and big wire from the batteries. I can't remember the amp draw for the solenoid vlv off hand, but seems like it would have been a little much forthe stock wiring.
I tried a redhead solenoid vlv originally, however, as the op mentioned, they require a pressure differential to work effectively. I Googled "ss solenoid vlv", and "solenoid vlv", and found a company that sells a few different brands of vlvs, several of which don't need a pressure differential to work. Think mine cost about 80$, and has a brass body.