Best ways to protect trailer's under carriage?

Don't drive on salt treated roads. The steel frame work and suspension are not protected against salt exposure. Other than that, keep corrosion at bay by wire brushing any heavy corroded areas and paint/touch up areas not still black with Rustoleum flat black including frame, propane plumbing, axles and springs. Use gloss black on bumper,and tougue/pin.
This keeps my rig protected and looking good.