I strongly recommend the following link for determining the true towing capability of the tow vehicle:
http://changingears.com/rv-sec-calc-trailer-weight-tt.shtml
The important thing to remember is the Combined Vehicle Weight Rating (weight of tow vehicle, trailer and everything in them) is the legal maximum weight. Even if improvements (airbags, HD springs/shocks, HD brakes, etc) are made to the vehicle the CVWR does not change. It is the legal max weight rating for the vehicle with trailer.
http://changingears.com/rv-sec-calc-trailer-weight-tt.shtml
The important thing to remember is the Combined Vehicle Weight Rating (weight of tow vehicle, trailer and everything in them) is the legal maximum weight. Even if improvements (airbags, HD springs/shocks, HD brakes, etc) are made to the vehicle the CVWR does not change. It is the legal max weight rating for the vehicle with trailer.