OK gang some facts on the things.
One manufacturer's web site: The Original Hott Rod
Similar products are or have been sold as "Hot Water Bandit" and "Hybrid Heat" by Camco.
You can get them for around $100 at any of the major RV supply places. AdventureRV, PPL, Tweety's.
Now, in principle I agree with Lewster that these things raise safety questions and lack the sort of agency certifications that we might like to see. My concerns in particular would regard the mounting of the thermostat (if it comes loose the element will run nonstop) and the attention to insulation, grounding, and leakage current. All these things are addressed by the agency certifications that these devices lack.
Potential buyers should also realize that these devices are around 400 watts, much less than the 1400 watt element present in an Atwood heater equipped from the factory for both gas and electric operation. So it will take maybe 5 hours to heat up a tank of cold water.
For most people the cost savings of using electric rather than propane when shore power is available will never pay for the initial cost. I don't believe there's really a fairness problem with campgrounds. They are free to set their charges as they see fit and some do meter the service or charge extra for electric heat or A/C.