Have you been underneath and visually checked that it only has one fuel tank? What chassis is the RV built on? A push/pull knob on the dash sounds like perhaps you have two fuel tanks with two fuel pumps or solenoid valves to switch from one to the other.
Many years ago I modified a school bus to use for road trips. It only had a 30 gallon tank. I got a second 30 gallon tank from a junked bus and added it underneath. A DPDT switch changed from one tank to the other, and it also changed the fuel gauge sender to monitor each tank's condition. Maybe you've got something similar.