He speaks the truth.. Test the water systems with what we call "pink Stuff" that is RV Antifreeze, take at least 10 gallons, to test with. Have him show you how to add it to the system, and pump it through, you can even heat it (hot antireeze) and then have him show you how to blow out the lines.
You will have a bit of work getting it all out of the system come spring,, but .. Well, it can be done.
NOTE: on my fresh water tank the drain is **NOT** in the bottom, this means about a gallon remained, I had to vacuum it out with a wet vac and a small hose inserted into the drain.