I have 2008 C4500 which has 40 gallon capacity fuel tanks(25 and 15). From what I have read in the manual,the tanks are connected and there is NOT a switch to switch from one tank to another when one runs out. Is this correct?

The PCM monitors the fuel level senders and transfers fuel via a transfer pump from the rear tank to the front tank as the front tank gets empty this system has had issues on my truck

OK,I guess thats why the manual says to fill the front tank first. What year is your truck? What model and what issues did you have with your tanks?

I have a 06 5500 the main issue I had was the senders got out of wack and it stopped transfering fuel and ran out in the middle of the interstate

Be interesting to find out exactly how the function is
I would assume only the front tank sender is live to the gage
I have two tank system but only one filler that feeds both tanks
Although I understand, how the one tank pumps to the other, I have never seen my gage increase
Does it empy the smaller tank while keeping the gage full?

Both senders measure level in the tanks. The PCM averages them and shows that on the gauge. If the transfer pump fails and the rear stays full you will run out of fuel and the gauge will say 1/2.

Both senders measure level in the tanks. The PCM averages them and shows that on the gauge. If the transfer pump fails and the rear stays full you will run out of fuel and the gauge will say 1/2.

That would suck bad . Running out fuel when gauge says half tank .

Hmmm,
Will a GM scan tool, read the senders individually?