Troubleshoot dual tanks

[rod1701]

While I’ve got the bed off, I need to fix my dual tank switch.

Done some reading that solenoid can be a failure point on these. But need a process for troubleshooting to see if it is switch, solenoid, or other.

Doesn’t help that gas gauge isn’t working and will need to test the sending units for both.

Any steps to help me narrow down the problem area without throwing a ton of uneceddary parts and $$$ at it?