I had the same problem when we got our rig. I took it to a place that understood alignment of Ford trucks and they set it and it made all the difference in the world. No special equipment, no shock replacement, just a proper alignment took care of the problem for us. AND, setting the front tires to the proper pressure.
Our Phoenix Cruiser is 24 feet long on a 350 and I run about 62 pounds in the front tires. Ford has the front end set for sort of general purposes when it leaves the factory. As I recall, the specs are very wide they provide for the alignment and a bit of positive castor seems to do the trick since it is now a motorhome and not just a bare railed truck.
Paul