The manual will reflect what total frontage area trailer max is for the vehicle, as far as air deflectors if the trailer front is not totally flat a deflector wont help much, the trailer is likely wider than the truck, and those outside edges catch wind, no deflector on top is gonna change that.
there are a few deflectors out there, but if it aint as wide as the trailer you arn't really doing anything. IMHO
2006 Expedition 4x4 w/HD tow package
5.4L 4 speed auto w/OD 3.73
tow rated 8200
2008 Cougar XLite 30FKS