Hi geof,
The class 8 tractors (and some foreign cars) have separate bulbs (filaments) and separate circuits for brakes and turn signals. RVs use the same bulb and filament. You can't simply tie the two together without causing undesirable conseqences back at the tractor (such as, illuminating the turn signals when the brakes are applied). A simple diode "OR" gate won't work either because the turn signal would stop flashing when the brakes are applied.
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