Back in the day with LP/120v water heaters we just turned on the LP about 10 minutes prior to using hot water. That left plenty for two or more showers even in very cold weather. We left the electric on as soon as we had water in the hot water heater. Do not operate it with out water or the element will burn out in a few minutes.
We usually ran out of the tank in cold weather as many camp grounds back then didn't have any freeze protection on the valves and we didn't have a heated hose at the time as well
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