Sway bars along with a Steering Stabilizer will run around $2500 to install. You can watch the Wynns video to see what kind of effect these additions make:
Gas RV Stabilizers – Do they Really Work
A cheaper modification folks add to avoid sway during high winds and passing trucks are AirTabs.
Airtab | Aerodynamic Fuel Savers | Welcome
These smooth out the airflow behind your rig to prevent a low pressure area from building up behind you. Reviews say they don't do much to improve mileage, but do wonders for swaying. Only problem is they look kind of dorky.
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