Volvo Vnm

Peterbilt we have has a ball valve on the bottom of each tank, might be something like that. Or, the crossover line may be plugged, if you haven’t added fuel in several years, that’s what I would check first. Take the cap off driver side tank and make sure it IS full, could have a bad gauge or sending unit

 

Try cracking the cap on the tank that draws low and see if it equalizes the fuel level between the two. As far as I know, this truck should draw from both tanks equally

I suspect one of your breather tubes are clogged

 

the vent is plugged on driver side tank. volvo draws fuel from both tanks and returns to both tanks, you will find splitter valve behind trans mounted on crossmember there will be a valve for supply splitter to both tanks and return splitter.