Just FYI in case you didn't know, 5 gals. of water will weigh right at 42 lbs.
Have you thought about using one of these Portable Telescoping Extension Collapsible Ladders? They come in various lengths, so you could get the one that right for your vehicle.
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http://www.amazon.com/Aluminum-Port...1457735878&sr=1-4&keywords=telescoping+ladder
It could be easily be stored in an exterior compartment where it would be secure from theft and out of the weather. Plus, you wouldn't have to climb UP to get the ladder DOWN so you can climb back UP!
Thank you for posting that , I'm getting one for sure