The alternator spins regardless of load so in theory having electric stuff on shouldnt make any difference as your already generating the power, for example you can play the radio have heated seats and demist your rear window with the engine off and no fuel being consumed as its off the battery, The aircon however doesnt work without the engine running as its only power is off the belt which only spins off the engine.
On that basis air con is driven and puts more load on the engine so you loose mpg however as the alt runs at 14.4v anyway an produces the power regardless of load, (rpm is the only exception as it doesnt spin to its full capacity when on tick over) swiching on your leccy toys shouldnt make any difference