There are certainly guys on here who know lots more than I do, but here's my $0.02:
Your cost up front would be pretty high (though probably not as bad as buying 3000 gals of diesel in one shot
): Each motor has a 3000 gal. fuel tank? Or 3000 gallons fills up all of them? At any rate, you'd have to buy propane tanks, then fill each of them up (to 80% capacity) @ somewhere in the neighborhood of $2.60/gal.
As much as you run those things, I'd say it's financially feasible to do it. You'll pay off the propane setup pretty quickly running that many hours. Guys are gaining what, 10% fuel economy running propane? 15%? Propane's obviously cheaper than diesel, and the way I think about it is like this: For every gallon of propane you buy, that's 1.1 - 1.5 gals of diesel you don't have to buy. So you begin paying it off instantly at the rate of $2/gal. If I was guessing, I'd say you'd pay off your investment and start putting money back in your pocket in the first 3000 gal diesel fuel-up.
Same as with an on-road setup, though, you want to make sure your EGTs aren't too high.