I am staying at a campground without water hookup and I am using my 12volt water pump. Does it drain your battery to leave the water pump in the "on" position or should you just turn it on whenever you are actually trying to run water?

The waterpump switch can be left on as the water pump should cut off after you turn the water off by a built-in pressure switch in the pump. Their will be no drain on the battery.

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Jim L & Faye
2805C
2003 Silvarado diesel crew cab
2017 Silvarado, crew cab 1500 6.2L gas w/ 8 speed tranny
central VA

The pump on indicator light will draw a small amount of current but not enough to worry about. For good insurance, turn off the pump when you are away just to make sure the unexpected leak doesn't happen. The same applies when hooked to city water, turn off the supply valve outside when you are away.

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My Great Wife Joyce
2018 Jayco Jay Flight 21QB
Formerly owned 1705T and 2310H
2012 F150 4X4 SuperCrew EcoBoost w/Leer Cap
Reese WD Strait-Line Hitch
Amateur Radio K3EXU