Check out this posting: Whats Your Real Weights? Truck and Trailer. The Real Deal
You will see in the posting, toward the bottom a web site that hopefully can be of help to you. I'll be watching this posting for addition help for me also.
http://fifthwheelst.com/before_you_buy.html
I am just like you and been trying to found out exactly what is the limit/maximum size 5th wheel I can pull safely. I have a 99 Dodge 250 cub cab 2/4 that weights 6540 lbs and the GVWR is 8,800, GCWR is 18,000. I am still trying to figure it out. I need help too.
