only pulling fuel from one tank

[Mr Hagg]

First remove the caps and look into both tanks to see if fuel is returning in both. If not then Id replace the return valve it works as a diverter valve. Its a equal flow system so say if the truck is on uneven ground more fuel will flow to the low side of the truck thus over night all fuel will be in that tank .
A few things to check:
Check the vent tube on both tanks. I have seen cases where it was stopped up and the tank would build pressure and would not let fuel enter once it had pressure.

Also like hhow55 said check the splitter valves. I have seen the return have trash in it and it would not return right.