On my Trek I noticed an improvement when I added the AirTabs to the rear of my coach. I also had four corner air bags which I could control from the dash to stiffen them up if there were heavy crosswinds. I had the True-center steering stabilizer which also provided some directional stability.
I never felt the need for a trac bar, or any other suspension mod. It is important to load your Trek as you would travel and get it weighed on all four corners or at least the front and rear axles. The rig will handle better if the front axle is loaded close to the MAX weight allowable and the tire air pressure adjusted for the weight.