dual tank problems

[79BIG10]

Here's a write up I did before on this subject. Hope this helps. Does your gauge move when you switch the tanks? Have you been kicking or moving anything near your fuse block panel? Just a few questions to help diagnose.
Now here's a couple ideas:
On your interior valve switch replace it. To me it sounds like you have a bad switch. I got mine with cover for like $10 from the dealer. Don't use the Auto Zone stuff. The dealer is a couple bucks more but why waste on something that isn't to GM specs? To take off the switch and cover the 2 nuts have to be taken off the rear of the cover behind the dash. Best way is to take out the ash tray and it's much eaier to get to then. Don't use the Auto Zone stuff. The dealer is a couple bucks more but why waste on something that isn't to GM specs? While you're there with the plug off let's see if we're getting power to where it needs to be.
From the 3 o'clock position this is #1 wire then so on till #5 for reference.
#1- Ignition wire (pink w/ black stripe)
#2- Valve wire (lt. green)
#3- Tank Aux. (lt. blue)
#4- Gauge (tan)
#5- Tank Main (tan w/ white)
#6- Not used
With the ignition on you should get power with the #1 slot. If not check the pink w/black wire by following it to the fuse panel. Mine was connected with a male spade connector to a IGN slot. This way it's only powered when ignition is on (neing not running). Also you should have a light when testing for the #4 gauge as well (assuming it the #1 is hot).

If this doesn't solve it then take a test light to your fuel valve itself. It is located on the passenger side close to where the cab and bed meet. There are 3 different setups on the valve. They are 3 ports (fuel) 1 wire, 6 ports 1 wire (mine) and 6 ports 6? wires. You need to see which one this is. On the 3 & 6 ports with 1 wire, the green wire, attaches to a bolt coming from the valve itself. This is what powers the valve to flip from the one tank sending hose to another. When you have the selector valve on the left side (with ignition switch on and for safety engine not running and emergency brake on) the green wire will have power (where test light comes in). When you have it on the right side of the switch the wire will have no power. This is why I like to run on my right tank more often than my left. Now if you don't have any power to that switch under the bed, you need to backtrack (next section). If you do have power then you need to test the valve itself. With the ignition on engine not running and emergency brake on (safety first) take the green wire with connector to the "bolt" that is the connection on the valve itself. As long as you have power (with the interior switch flipped to the left side) you should hear the valve turn to the left tank. It sounds like a "click" almost. If you have power to the green wire but the valve is not changing tanks this is your problem. Replace the valve (I got mine for $70 from a AC Delco parts warehouse) and while you're there replace all the fuel lines as well. It's cheap insurance. Just make sure to label the hoses so you know which goes back on where. With 6 hoses on mine it was a job to make sure everything was on right.
On the firewall driver side look for a 4 wire harness with a bulk connector (looks like a connector that has a rubber boot) It will have 2 tan wires a green and a lt blue wire. This is where the tanks and selector valve wire come from the interior switch. If this is tight and no corrosion then we'll need to rethink this. Hopefully this works.
If you have a 6 wire then let me know and I'll try to find a spec for that then. I have't messed with that wire harness yet at the tank selector valve itself.