They've covered the possibilities... voltage, thermostat, heating element...
Running the electric heater with no water in tank can overheat/burn out the element. Never know what the previous owner did.
Cheapest way to run is electric if you aren't paying the electric bill - ie - included in camping rates... Otherwise depends on local rates and gas prices.
Fastest way is to use both... also can allow for longer showers.
Not yet retired...
Florida (Michigan transplant)
2014 Hurricane 32A
2000 Infinity (previous)