I installed a recirc line in my motor homne and am very happy with it. I put a 'T' in the hot water supply line to the kitchen faucet (this is the furthest faucet from my hot water tank). The water from the 'T' is plumbed through a plastic ball valve inside the door of the faucet cabinet and then routed through a hole in the floor to a 'T' at the outlet of my fresh water tank. When you open the ball valve the cool water in the hot water line is pumped by either city water pressure or by the fresh water electric pump back into the outlet line of the fresh water tank where there is no pressure. It only takes 10-15 seconds and then you turn off the ball valve. The water from the kitchen faucet is now immmediately hot and not a drop goes into the grey tank waiting for hot water. One small drawback is that when hooked up to city water and while the ball valve is open you are adding fresh water to you tank. If you were to turn on the valve and walk away you would over fill your fresh tank at some point. Never happened so far. It would just spill out the vent tube anyway.
