I love that idea Randy, but I do have one slight concern (the same concern I had when I was thinking of mounting a water tank to my roof), which is how will a bunch of sloshing liquid mounted that high above the center of gravity affect the vehicle while wheeling, especially if you get into a particularly hairy situation like the one I got in to up on Cleghorn a couple months back.
I took a bad line and ended up sliding into a rut and came to a stop with about 30 or 35 degrees of roll toward the passenger side and was well up on 3 or even 2 wheels. PainRoller and FixItGuy were quick to run over and jump on the driver's side to keep me from going all the way over. Simply having an extra 70 lbs (12 gallons of gas) on the roof probably wouldn't have been enough to send me tumbling all the way over in that incident, but having 50 lbs (8.5ish gallons) shift rapidly to that side of the roof while I was tipping would certainly have done the trick.
I've often wondered about the effects of slosh and the way it shifts weight. It doesn't seem to be an issue at all if the weight is low and you're under normal driving conditions - hell, they don't baffle stock fuel tanks or even large fuel tanker trucks that you see on the highway. But the moment you move that weight up high and insert yourself into a situation where you're guaranteed to be pitching and rolling a lot, I wonder what could happen. Probably a very low risk, but seems to me that your best bet would be to fill it all the way up every time you have it on your roof so the weight doesn't shift, and then to empty it fully when you do use it.
