What are the dimensions (is it tall or is it squat)
I would use a tray and collar system. A lipped tray for the bottom (which you bolt to the bed) and a collar for the top with some loops so you can strap it down to the deck with some eye bolts. kinda like this
Check with a local fabrication shop if you can't find something already manufactured. They should be able to do itpretty cheap and quick (with in a day or 2).
Definately keep it up by the head board (one less direction that it can move). As for the weight, it will continually drop as you are using it, unless you just give joy rides to 900 lbs of water

Just make sure that you position the tank over the ribs or have extra put on to support the weight.
P.S.
No picking on the picture, graphic arts isn't my thing LOL
Good luck
"J"