Be advised that big temp swings will change the pressure of an organic gas (air) substantially, maybe as much as a couple/few hundred psi in a 4500psi tank,, from in the sun at 95° to a cool 40° night, or even sitting in the snow at below freezing. The above tip by FF to store in the shade and up off the ground are good ones,
I'll add, might want to make a rack to secure/strap them down firmly in a horizontally position with the valve protected/pointed out of harms way just in case something falls on it. That way it can't tip over and fall down maybe hitting/damaging the valve. Unlike many DOT approved HP gas storage tanks that have big caps that screw over the valve to protect the valve, ours do not, so keep that in mind when you choose where to store them...Are these cf scba or scuba steel/alum tanks..?