There is a lot of MIS-information here and a lot of GOOD information. There are MANY ½t trucks very capable of towing a 5th wheel and staying within the specs of the truck. The 5th wheel tow rating on them is very different than the trailer tow rating, which you quoted. also, ALL truck manufacturers have increased the truck payload over the past couple years to the point that with the proper ½t, it is not any problem.
One of the gooseneck ranch trailers I tow in the mountains with my ½t weighs 10K when loaded and I have NO problem towing OR stopping, as the GM NHT package has the power and the brakes to tow 10K+.
Trailer manufacturers have worked very hard at reducing the weight of their units and still maintain safety. Look for the ½t 5er that will work for you, then look at the new ½t trucks and find one that has the specs for your trailer. The combination you wasn't is out there, so don't get discouraged or think you have to spend a fortune to get what you want.
