If it works when you get it, it'll most likely make it through the week. They're pretty reliable and have hardly any moving parts. They're about $75 - $100 new. But the world is full of campers that leak and rot and can be scavenged for treasure.
I'd go with that setup for lots of reasons on top of being available free or cheap.
It's made for exactly what you want, and the built in on/off valve on the camper shower wand makes it easy to get the most out of your water supply.
They're small, water hoses clamp right on, wiring is easy and safe (it's only 12 volts) and it puts out a pretty good water stream.
If you wanted to get fancy, you could scavenge the propane fired hot-water heater from whatever camper donates it's other parts... and even the 30 or 40 gallon fresh water tank it probably has.
But that might be "snowballing" the whole originally-simple idea... doesn't that always happen!!
GreyCoyote: "At this rate it wont be long before he is Admiral Fukkit."