Contact Caterpillar. They have units up to 6.7MW; I'm sure that they can reconfigure one of their gaseous fueled units to run on propane instead of natural gas or one of the new biofuels. Your biggest problem will likely be keeping enough propane on hand to keep the unit running...these things are very thirsty. Be sure to size it so that it can handle starting currents of your larger motors and elevators. Remember that if this is going to be your sole power source for more than the duration of a minor outage you will need to use the rating for 'prime power' instead of 'standby' or 'emergency'.
I don't know how much Propane it would use, but Cat's 375KW natural gas genset drinks 5141 standard cubic feet per hour or just under 1.5 cubic feet per second. So if you wanted to run for an extended period that's almost 125,000 scf per day. Someone's going to need a very big storage tank.
All the math looks good, only problen is these engines burn vapor, not liquid, as the level in the tank goes down the surface area of the pool gets smaller and cannot vaporize enough gas fast enough to feed the unit under load. Likely as not you will need 2 1000 gallon tanks to keep up. Also if this is a Life Safety situation you do not have a prayer of getting a gas generator started and up to power in 3 seconds. Look into dual fuel units, they burn a combination of diesel and propane, they start quick on diesel and use propane to help lower the diesel consumption.
Life is tough, Life is tougher when you are stupid