I think it would be fairly straight forward to mount, but I wouldn't put that weight and additional height up there. Mount it sort of like solar panels but stronger. Run 3 - 4 channels of Unistrut across four roof members to spread the weight then strap it down with two stainless straps over the tank in the groves made for the strapping. The tank mfg should be able to provide the strap kit.
You'll have at least the 280#s up there that mabynack estimated.
I don't see a problem with using it for drinking water if you keep it clean. Your standard MH water tank is down inside of the MH where it's not subject to direct sunlight. Sitting in the hot sun with moisture in the tank even after draining, is going to grow "things" in it.
Bill