Bacchus, does that 3 way valve prevent AF from flowing backwards into the fresh water tank?
Yes, a 3 way is much more than just a T the 2 valve position are...
1- allows flow from tanknto pump inlet
2 - allows flow from the hose / hose port to the pump inlet
If you switch the valve back to normal tank supply with pink in the pump inlet you can get a small amount (tubing contents between pump & 3 way valve) that might drain back to the tank. If that is not desired there are ways to avoid it... use compressed air to blow the line out; leave the valvenin winterize position until spring and just run a jug of fresh water thru the pump before switching back to normal tank position / flow.
The water heater bypass kits contain 2 of these valves and a connector hose that connects the 2 side "bypass" ports. Drain the htr tank ( I use comp air and a hose overboard) throw the WH valves to bypass before using the pump inlet 3 way valve a hose for supplying pink AF to the FW system. By openning all H & C faucets, showers, etc (don't forget windshield washers if fed from FW system) and FW winterizing done. I leave mine in bypass positions until spring and usually hook up to shore water to flush all the H & C lines well BEFORE turning power on to the water htr. I'll then switch from bypass to normal WH flow and do a little more flushing.
The key to avoiding any pink AF in tank is to make sure you have a way to completely drain the tanks. I installed a plugged PVC T in the low point in line than connects my 2 FW tanks to easily & positively drain my tanks.
I have read & agree that a small amount of water in a tank is not a problem for the tank... my concern I didn't want to learn the outcome hard way is... if there is water in a tank outlet or cross connect fitting will it burst the fitting or us there enough room for expansion back into the near empty tank??? I'd rather not find out the hard way so I make sure my tanks are empty. I have sometimes left the T plug out so that if water remains in the tank and moves during haul out ( I winterize in the water before the yard pulls the boat and winterize engine)
That's the unabridged story but hope it helps.
Installing the valves is a little more work but only once and then winterizing, flushing is EZ with only need to run valves.
I've modified my AC cooling system (based on suggestion by a TFer that was smarter than I was) to make winterizing EZ. Happy to share if there is interest in AC cooling syste... just let me know.