Agree on all above. To get into troubleshooting mode, you are likely looking at an issue on Driver tank only, possibly obstructed pickup, possibly cracked hose sucking air, possibly a combination, and possibly something different. Your "known good" point is probably the line leaving the switching valve, everything backwards to the driver tank cap needs to be checked in whatever way is most appropriate until you find the culprit.
My preferred route is to find something easy access, roughly in the middle of the system and test there. If the problem stops, you have a new "known good" point, and you test at the next easiest point in the middle, and so on.
For this, I'd start at the 3/8 line going into your selector valve from the driver tank like Lego said. If it works fine, you can try just replacing the hose from there to the sending unit. If that's it, you found your air leak and party on. If not, you are probably looking at dropping the tank and your issue should be relatively easy to find from there. If the new line and gas can doesn't fix it, then you are probably looking at the selector valve and/or connections to/from it. Replace more hose and clamps (since that's never a bad idea on an old truck) and see when it starts working.
