our 1999 34' has the 'gauge' type and pressing the button you mention, gives a fairly accurate LP fuel level. If your tanks were replaced with NON gauged type, then someone may have 'tucked' the wire ends somewhere... or cut them.. Have a look under the frame...
I added a SAE insulated and keyed two wire connectors.. look like small trailer lighting connections.. makes it easy to disconnect the tanks for refill. I believe LP1 is the Street side, LP2 is the Curb side..
We also have the same dual supply, manual selection valve and regulator assembly you do. The 'lever' points to the active supply input. You can install an "automatic switch over" which will swap to the full tank when the pressure drops on the active tank.
You CAN buy the tanks with the sensors from AS dealer ... but there is a far cheaper way... PM me for details.. (I have no financial or personal interest in the tank company.. just trying to save folks $$ and increase satisfaction)
Peace and Blessings..