De-Winterizing

That's a pure spitball, but I'd put it up against the other possibilities.

In marinas the more that current leakage occurs, the faster that galvanic corrosion takes place.  In fresh water there are less minerals that can conduct electricity, but that doesn't mean that there are none.  The more minerals in fresh water the faster the corrosion can take place.  If you are running pure Sierra Nevada snow melt water in your fresh water system of your Pod, I suspect you will get years of use from your anode rod.  [Which has been my experience as this is what I usually use to fill our water tank and our anode rod has been in the water heater 3 years and is barely corroded.]  If you draw your water from a place that provides water for a mineral bath resort, I'll bet you will find your anode rod will get used up much faster, especially if there is current introduced from the place where the burned up calrod used to be attached.

But, I will certainly defer to corrosion experts who know far, far more than I do about the process of galvanic corrosion.  I did google the issue, though, and found several sites that discuss the quetion.  Basically, they say that when dissimilar metals are exposed to a conductive liquid and some kind of electrical contact, galvanic corrosion will take place.  The idea of using aluminum, zinc, or magnesium on sacrificial anode rods is that they are more susceptible to corrosion, thus, as long as they are present they will corrode first.  Here is one of the articles:  https://www.assda.asn.au/technical-info/technical-faqs/galvanicdissimilar-metal-corrosion

The difference between most people's water heater anode rod durability and that of the water heater in the Pod is exposure to electric current.  Most gas water heaters are not connected to a 120vac system unless there is a recirculating pump.  Thus, the electricity they get is what gets in through the ground, conductive water and or metal pipes.  [Another advantage to pex plumbing, I suppose.]  The less electric current exposure, the more slowly the anode rod is consumed.  In the Pod, on the other hand, there is 12vac supplied to the tank via the calrod and even though the current leakage may be minimal, it's still there.  Add high mineral [conductive] water and you'll be replacing anode rods regularly, but at least your water heater is protected.  I suppose the next question is how big are the anode rods in an electric water heater and how long do they last.  I asked Google and she wouldn't tell me.

In marinas, where some have a lot of stray current in the water while others don't, you see a big difference in how fast the sacrificial zincs protecting the propellers are eaten up.  Some make it a season or two, while others can't last even six months.  

The illustration is accurate for any hot water heater anode as to showing what used up looks like.

TT