Soundguy,
I have a '99 550 auto 7.3 4x4 and a '05 550 auto 6.0 4x4 that I have had since new. These are good trucks for pulling a 5th wheel or gooseneck, one thing you have to watch with trailer selection is the weight of the trailer itself. I mean you can get a trailer that is so big and heavy that it reduces your payload capacity so much that you get in a catch22.
I pull my small tractor and backhoe with my service truck on a 12k bumperpull trailer most of the time. When I need to go farther and carry more than will fit on the service truck I also have a f350 diesel with a flatbed. I can load some implements on the truck first and then load the trailer.
At some point I may swap bodies and put the flatbed on the newer 550 as it is rated for 33,000 gcwr, then pull a 14k bumperpull trailer. I have a tandew 25GN trailer now that sits most of the time because it is so heavy. I think the truck and gn trailer empty are close to 13,000-14,000 lbs. Whereas I can use a F350 or F550 with a 14,000 lb bumperpull with 20' floor which weighs about 11,000 lbs empty, carry 11,000 lbs payload on the trailer for a total weight of 22,000 lbs. This means I can carry almost 4000 lbs on the flatbed truck and 11,000 lbs on the trailer and still be below 26,000 lbs.
Hope some of this helps you decide what to do.
FWIW, I have three 7.3's and two 6.0's, never put wrenches to the 7.3 motors but have replaced the shortblock in the newest 6.0 and worked on the other 6.0 too. None of these trucks have over 75,000 miles either.
