[JRTJH]

Assuming it didn't leak before you installed the second tank, I'd first check to make sure the plumbing for the new tank is secure where it enters the OEM fuel system. If that is not the source of the leak, then, since it's a gravity fill system, it sounds like the factory fuel tank is overfilling and the excess is leaking out "somewhere". I'd check the fuel cap to make sure it's not leaking both from the cap and from where the filler pipe attaches behind the bed sidewall (there are three bolts that hold the hose to the back side of where the cap screws into the filler pipe). Then I'd check all the clamps on the overflow hose. Here's a picture of the filler pipe. As you can see, there are 3 bolts holding the top in place and several clamps (either 4 or 5, depending on version of the filler pipe) that secure the hose to the tank.

Most of the above is fairly easy to access with a creeper and some smooth concrete to roll around under the truck. There's almost enough room to roll over under there, and it's fairly open around the fuel tank.