Everything in the car that uses power will reduce your mpg, but you need to be realistic. Your engine is generating about 10000 W of power to keep you at a constant 60 mph, so a set of lights using 100 W is going to make about 1% difference.
When travelling at a constant speed (eg on motorways) slower always uses less fuel. You must average your mpg over a long distance though, even very slight inclines or wind can affect the numbers by a large amount. Also the fuel your engine needs to develop to keep a constant speed is roughly proportional to your speed squared, so 85 mph is going to need twice as much fuel as 60 mph, and 95 is going to need twice as much as 65.
Off the motorway the speeds are
lower so don't make so much difference, the most important thing becomes how often you brake. Braking simply wastes fuel, it's far better to lift off the accelerator 300 metres from a corner rather than braking 50 metres from it - it uses 250 metres less of fuel just for that one corner alone. Leave a bigger gap to the car in front and try to anticipate thing further in advance to avoid having to brake except when only absolutely necessary.
Lower RPM doesn't automatically equal less fuel being used, the efficiency of the engine depends on both RPM and torque (throttle position) in a non-trivial relationship (I have a chart from a petrol engine if anyone is interested). The 6th gear ratio will have been chosen to put the engine at the most economical operating point at 130 km/hr (or whatever speed they think will be most common).