A couple years ago, I replaced my Raritan. Not sure how good the tank was but the skin had nearly rusted to dust. Unfortunately, the nearest replacement had a minor design change that made it just too big to fit into the lazerette. However, was able to locate one of the box shaped ones with an aluminum tank and a stainless steel skin. Forget the brand but I think the same thing is being marketed by West Marine as a Force 10.
One drawback of aluminum is the possibility of galvanic corrosion to the tank. The installation manual specifically said that warranty was voided if the shore power system did not have a galvanic isolator. Not a problem for me since I'm on a mooring and even if I'm at a transient dock with power, I don't think I've plugged in the boat for years.
As for the coolant, yes when the engine is running and the water pressure is off, engine loop coolant is at a higher pressure and can conceivably leak into the hot water tank. Although hot water is mostly for washing, I don't want the most remote chance of injesting ethylene glycol. I always use a propylene glycol (non-toxic) based coolant. Last stuff I picked up was Prestone low-tox. Used to be carried at Wal-Mart but couldn't find it there last time. Not every auto parts store has it. You may have to check two or three different stores. Still, worth it for the piece of mind and only a buck or two more than a gallon of ethylene glycol.