Unfortunately once rust starts there's no way to really stop it without major surgery. You can try a rust inhibiting primer or there's a rust converting type of coating that's available from the local body supply shop. Just sand the affected area, front and back, as good as possible then brush on the coating. It'll convert the rust by stopping the oxidation that causes it. If the metal is coming apart, you can then repair the area with fiberglass, prime and repaint. If it's still somewhat solid with mostly surface rust, just sand it smooth, apply the coating, prime and repaint. It's not a permanent fix but it should buy a couple more years for your rig.