I've had propane on my truck for over two years without any adverse effects. I've got a 9.9 gallon Manchester tank mounted under the bed, and it lasts me 2000-2500 miles between fillups. Here is a pic.
lukecliine is right, though. Diesels and propane don't get along well if you turn the propane up too high. I've got mine set to give about 80 additional hp. It's the guys trying to squeeze out 100+ more than run into trouble.