The tanks are at the same level, both are mounted between the frame rails, but the new rear tank holds about 10 gallons more which I believe starts the flow to the smaller front tank. Then a siphon effect is created and out the front fill port it goes. The pump does prime as before as long as there is water in both tanks. I already ordered swing check valves so I will try that first, otherwise I will go with a valve set-up. Ain't nothing ever easy!