If you go by the postings on this and other forums, there are not many 40' 5th wheels that are suitable to be towed with a SWR 1 ton. The F350's I have looked at appear to have a cargo weight of about 3,600#. I would guess that GM and Dodge are about the same, but I haven't confirmed that. This being the case, when you add 400# of people, tools etc. to the truck you are at 3,200 max for a pin weight. Now use the formula which seems to be the norm on most forums of 80% total capacity and you have an allowable pin weight of about 2,500#. Most 40' 5th wheels that I have looked at have an empty pin weight of 2,500 to 2,600. Loaded you are going to be looking at 3,200 to 3,500 I would guess. Based on this conventional forum wisdom you really can't pull a 40' 5th wheel with a 1 ton SRW if you go by the specs and formulas.