You lost me on this one. Are you talking about using diesel glow plugs for heating elements?

A: Glow plugs are not designed for immersion. They would quickly be covered with corrosion and heating would be almost nil.

B: Glow plugs are designed for a 30 second maximum "on" time. Nowhere near enough time to heat any amount of water.

C: 30 amps or so of draw would quickly kill any battery by the time you got a gallon or so of water hot.

Stick with the propane heaters.