Arrgh...
Looking into the Magnesium vs Zinc thing. What I
know is that Magnesium anodes can react with sulphates in the water to produce Hydrogen Sulphate (aka stink). I
believe that this is
less of an issue with zinc.
Digging deeper; I looked through the Isotemp website for user manuals. Here is the link:
Isotemp Slim & Basic
What I found after skimming a couple of them is that there is NO reference to the anode for an IsoTemp Water Heater

. This leads me to conclude that there is no anode! ("There is no spoon."

)
Theory: there is bacteria in your fresh water tank, and the temp (>160°F) is killing them in the water heater. As the bacteria decompose in your HW tank, excuse me...

, other bacteria are producing hydrogen sulfide.
Possible solution: Chlorinate your entire freshwater system, and then flush with CLEAN water from a different source than you usually use.
I have used swimming pool shock for my (new to me) boat, which had sat unused with full water tanks all summer long. Swimming pool shock is (
calcium hypochlorate). Others prefer bleach (
sodium hypochlorate) to chlorinate the system. If you want my rationale, follow the links that I provided. What ever you use, hit it hard, and flush it out several times before using.
Good luck!