How heavy is your 5th wheel. I just sold our 2002 F350 7.3 dually and it had 123,000 miles on it with Pacbrake and upgraded transmission cooler. That 7.3 diesel is a great engine and you would be shocked at how many are still out there with even fewer miles on them than mine had. The weak spot is the transmission. But if you don't overheat it, that's where the larger cooler comes in, it will last forever as well in my opinion. Low spot if you want to call it that is it's not capable of pulling/ 12 or 13000 lb. The guy I sold it to I'm afraid is going to destroy it if he hasn't done so already. He has a huge 40 footer and I just questioned him if he thought that was okay as P Ender stood though towing capacity of my truck.