I don't know of any vehicle (as equipped from the factory) that increases the idle speed to compensate for a large draw on the battery. If the alternator starts bogging the engine down then the computer should compensate to maintain the set idle speed but not make it increase.
It's easy to find out if the alternator will put out enough current at idle to run the light. With the light turned on and the engine at idle check the voltage at the battery. If the voltage is 12.6 or higher then alternator is putting out enough current to run the electrical loads. If the voltage falls below 12.6 volts the alternator is not putting out enough current to run the electrical loads.
If your alternator doesn't put out enough at idle you have a couple of options install a high idle device to raise the RPM. Install a larger amp alternator, there may be a larger one available from your parts store or you can have your alternator rebuilt to a higher amp version.
Some other options to consider:
Hard wire the inverter and put an outlet that's easy to access from the bed of the truck. Then you could just clamp the light on the roll bar (or wherever) and plug it in whenever you wanted to used it. This would eliminate any issues with vibration causing the light to prematurely fail.
With the price of fuel and the unnecessary wear on the engine you could consider a small generator. Honda makes a small quiet 1000 watt generator that would work great for this application. Some of them have a Halogen flood light attached already.
As already suggested you could use a 12 volt light to get the same effect. LED lights are expensive but they will have a lower amp draw for the same amount of light.