x2 on DrewE's recommendation, just turn off the main pump.
I don't see a way to do this without a solenoid valve, using a secondary pump or not. Once you open the closed-loop system by putting a return line from your hot water heater into your fresh water tank, when your main pump turns on, it's going to lose all its pressure into that return line. Your fixtures won't work, or will work very poorly.
On my system, I have no return line. My little pump just circulates hot water through the water heater, using the existing hot and cold lines. It only keeps the lines from freezing, the tank itself is inside the cabin and is heated by the furnace.
