There were factory installed aux tanks. Either a behind the axle N18 or a driver's side forward under bed NL2. These would be on your SPID. Aftermarket and dealer sold/installed tanks are more often found with through bedside filler doors or wheel well fillers.
Here are a couple of pictures to get you started.
The entire NL2 tank installation is covered in the assembly manual.
The sender picture is the factory NL2 aux tank sender and it is application specific.
We also hashed a lot out on a WTB thread of Jockos that is worth reading through.
http://67-72chevytrucks.com/vboard/s...d.php?t=560607