I have a Jamboree motorhome on a 1989 E350 Ford chassis, 7.5L engine. It has a fuel delivery problem when in hot weather. While tracking this problem down, I ran across a plastic gizmo up near the fuel injector fuel pump, mounted on the frame. The fuel, both coming from the tank and also the fuel returning to the tank go through this thing. What is it? It's not aftermarket, it's factory installed. It's a little larger than a baseball.

 

There is a filter in there. Most of the times I have seen them there were two fuel tanks on the vehicle and it worked with the fuel selector valve. Do you have dual tanks on the Vehicle?