I think you have it wrong way round Tim, you should be sucking the fumes out of the road draft and allowing fresh air into the oil fill cap - that's the way the factory does it and why it's called "positive crankcase ventilation" .... when moving, the road draft tube draws air out, so you'll only have air in when sitting at an idle, and there's not really much in the way of fumes in the top cover
same for you Dave, the PCV won't have any effect on what you smell after the engine is shut off, and any vapor you see from the rocker cover will be primarily water vapor - more likely the garage smells like the exhaust fumes and leaking fluids evaporating from the outside of the hot engine
Bill
