I have ~200W of solar on the roof and on a sunny day my house batteries are usually 100% charged by late morning... so I decided to put the extra solar energy to good use and I built a 150W 12Vdc heating element that screws into the anode port on my Suburban hot water heater. This heating element is operated as a "diversion load" so it only powered after the batteries are fully charged. Because the wattage is low, it only heats the water in the 6gal tank at a rate of 10ºF/hour, but hey... FREE hot water
While I was at it, I included a 120Vac 500W heating element (30ºF/hr) so I can heat the tank when I'm connected to shore power or via the inverter while driving. Of course, each heating element is controlled by its own thermostat.
To the best of my knowledge this is the only dual voltage RV water heater ever produced... of course if I need hot water in a hurry it operates quite nicely on LPG too.
Photos are below.