85 E350 Ford not switching from Fnt to RR fuel tank

My moms 85 Ford E350 chassis motor home wont switch to the rear gas tank. When the you flip the selector switch at the dash, you can hear a click from the transfer switch on the frame. The rear tank is full of gas. The generator's gas feed comes out of a tee on the rear tank line (between the tank and transfer switch), and the generator runs fine. The transfer switch is a 6 hose unit. I have heard that not switching between tanks is caused by the transfer switch. Any help would be great - espically how to test out or confirm that the the switch is good or bad!