There are some over on the Grand Design forum who are towing the lighter (<10,1000 lbs.) fifth wheels with F150s equipped with the Max Tow and Max Payload packages and the V-6 EcoBoost. A few things you need to research/be aware of:
1) You need to know the actual weight of your truck with full fuel, all passengers and the weight of a fifth wheel hitch. Subtract the scale numbers (front and rear axles) from the GVWRs for each axle listed on the truck's door sticker. You'll be surprised at how little payload you have left over with a 1/2-ton 4WD.
2) The pin weights manufacturers list for fifth wheels are empty weights. Use 23% of the trailer's GVWR off its weight a loading sticker to estimate loaded pin weight. NEVER use empty weights for anything.
3) Unless you plan on traveling in mountain country, engines aren't as important as weight capacities. The 5.4 will pull almost anything your truck can handle from a weight point of view... but the mileage will suck and you'll have high RPMs on grades. Make sure the truck is equipped with a good transmission cooler.
Rob