Larry Cad. One Thing Winne/Itasca did starting in about 1981 was use Atwood Water Heaters with Motoraid on all their Motorhomes. I KNOW this because I started in 1979 as a Tech and we sold Winne/Itasca until we dropped them in 1989. AND I replaced a LOT of Atwood Motoraid inner tanks over those years due to freeze damage. SO, I KNOW how they are constructed. To your point. SMELL would be the LEAST of my worries in your scenario. If you got smell from the hot water, then you had AUTOMOTIVE ANTIFREEZE contaminating the fresh water system(Deadly to ingest). I don't doubt you may have had a smell, but it was NOT from Coolant getting into the tank or Water piping system. AND, it would be at EVERY Faucet, not just the Shower head. Most common Water Heater odors problems are rotten egg smell from OLD water left in the tank or a high mineral content of the water(usually well water). Doug