By how much will running a 200watt thing (ex. small tv) in a vehicle lower the fuel economy?

well, an alternator induces drag when being used. If you need a high-amperage device to operate, then the alternator will be in Charge mode more than with regular use. It will cause more drag on the engine, and reduce gas mileage. How much? Probably not much, 1-2 mpg at most, but I doubt that high even. Probably less than an A/C unit would drop mileage. A/C units induce mechanical drag more so than the alternator, so you see a bigger drop in gas mileage using A/C. Go ahead and watch TV.....<grin>

Will not lower fuel economy. Electrical application in your example is not related to fuel consumption.

its negligable , since the alternator is already producing more than enough power to the equipment you already have installed stock in your car . doing that won't have an effect at all