If you are talking an Atwood HW heater, still using it on LP gas mode but tanks shut off, but the switch for electric ignition is still "on", for the gas mode then
When the supply line runs out of gas and the unit was heating or trying to heat, (will not take very long, a minute or less,) the unit will sense there is no flame. When the no flame sensor detects it is not lit, it tries 3 times like Mike stated to relight. After the 3rd time it will shut down on safety and not retry again until you turn the heater off and back on to reset.
If the 12 volts is off, battery unhooked, battery disconnect switch turned off or the 120 VAC convertor is off like the shore line cord pulled out, then it will not matter as there is no 12 volts to even attempt a relight. You did not mention if the 12 volts was on or not. The HW heater must have 12 VDC to operate the controls. If you have the 120 volt electric element option even that will not turn on if there is no 12 volts DC on the newer units. However some of the older 120 VAC element units had there own little system for the electric element. They could of run on 120 VAC all by themselves.
If you where on gas mode only, then odds are very high the unit just went out on safety and will shut down within minutes of trying to relight.
If you where talking the 120 Volt AC element, then that can be more of an issue being left on depending on how man weeks or months we are talking about. Let us know if you are taking the 120 electric element and if you have any water in the heater.
Hope this helps
Current Sunlines: 2004 T310SR, 2004 T1950, 2004 T2475, 2007 T2499, 2004 T317SR
Prior Sunlines: 2004 T2499 - Fern Blue
2005 Ford F350 Lariat, 6.8L V10 W/ 4.10 rear axle, CC, Short Bed, SRW. Reese HP trunnion bar hitch W/ HP DC
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