First question, how many amp-hours is your truck's starting battery? As a guess, probably no more than 70 Amp Hours. 90 watts is going to be 7.5 amps at 12 volts. So in something like 9 hours you'd exhaust that battery totally. Of course, it won't get that much because the rated capacity is calculated on a 20 hour discharge time, so maybe you'd get 6 hours to total discharge. A little less if the battery is not new, even less if it isn't fully charged, which automotive systems rarely do anyway.
Now there's no way a nearly fully discharged battery is going to be able to crank your starter. MAYBE a 50% discharged battery could do that, Maybe not. That says at best, you might get 3 hours before you couldn't start your truck, but there's one more factor to consider here: starting batteries are not designed to be discharged that deep. Running that inverter for anything like 3 hours is going to completely kill your battery long before it would otherwise die.
So I'm going to make a suggestion to ya: buy a deep-cycle marine battery just for this purpose, and charge it up on a battery charger in the house before you go tailgating. It'll run your stuff longer, will deal with deeper draw-downs without killing the battery, and you'll never have to worry about being able to start your transportation.
