Some links for you:
BLT Offroad Avalanche
Expeditionswest Tacoma
ThunderII - S-10 crew cab auxiliary fuel tanks
The Avalanche is my current truck. Scott Brady did the Tacoma. The S-10 was mine and inspired by Scott's install.
Notes:
1- The tee in the vent line doesn't work. It floods the evaporative emissions system. (Learned from experience.)
2- I use a momentary switch for fuel transfer. It does not take long. I add fuel around 3/4 in the main and keep it hovering between 1/2 and 7/8 most of the time. The momentary switch eliminates the possibility someone accidentally turning it on, or bumping it. This worked for both my installs and I do not see any need to change even today.
3- Gravity feed doesn't work. It will over fill the main tank and flood the evap system too. (learned from experience) An electric tank valve solves that problem as can be seen in my S-10 install.
4- The stock evaporative emissions system on my vehicles were designed to handle 100 gallons or slightly more. It can handle the added tank just fine and not over whelm the system. Mine is tied to the stock system. Keep in mind that does not mean it would pass an inspection if that is a requirement for your locale.
#Tip:#
Use hard fuel line as much as possible. If that is not an option use braided stainless fuel line. Hoses exposed underneath deteriorate at a faster rate than in the engine compartment. Spending that little extra in the install, save replacement and dangerous leaks later (learned from experience)
I have been using these systems for just over a decade now. The latest in the Avalanche is the best of everything in one. Lessons learned and improvements made.
Both fills in one location-
Dakota Digital fuel gauge reads in gallons and easy to calibrate for the fuel tank size as well as working with numerous types of sender-
Note the LED indicator and momentary switch. The LED was added later. It is a visual indicator that the relay is indeed energized. Never a problem, but comforting to me
I know it is working.
Tank tucks up nice underneath leaving room to stow the bridging ladders-
This truck carries just over 49 gallons of gasoline when full. That gives me a conservative range of 750 miles. I usually fill up at between 450-500 miles.
I have found nothing I would change and it has been wonderful. Those links offer many ideas. I knew of a few more, but don't see the old links active any more. All this started a dozen years ago.
Good luck with your installs people!