I would like to hear from a real mechanic about my truck.

Ok so here's the story. I ran out of fuel one day and learned my truck was only getting fuel from one tank. After a day and a half at the ship, my company decided that I should run on one tank and they routed me down to Phoenix so their "mechanics" would fix my truck. After I got my truck back and did my 34. I started the day with a little over half a tank. Over the course of 2 hours or so, my fuel gauge went back to full. On a hunch I stopped and sure enough, my passenger side was almost empty and my driver side full. So I brought the truck back to the yard and reported it to the mechanic and he says "oh, the tanks are designed to drain fuel only one at a time. When the first runs out, it will switch to the second one." I am no mechanic, but that sounds stupidly wrong. Is he full of #### or is that how it workd

 

It's a 2017 freightliner cascadia. I was guessing the fuel gauge was set to the driver side.