Atwood heaters are plastic (coated?). Suburban heaters are steel and do need sacrificial anodes. On mine the drain plug includes a sacrificial zinc element, and I have a replacement waiting to replace the one that's in the tank now. It dates from the original purchase in 2002 and is still only about half gone. I expect a couple more years before I need to replace it as we have very benign city water in Anacortes (one of the top five for quality nationwide) and we always leave home with a full water tank.
Most CGs in the western states have pretty good quality water. It's limestone (calcium carbonate) in the mountains where the reservoirs are that cause hard water problems. Our mountains are all granite or volcanic in origin.
