I tried one of those big deflectors on my TV years ago and would check my mileage both with and without. There was no discernable difference - just something else to have to mess with and store.
Chuck, Ann & the Boys (a pair of large labradoodles)
2013 F350 Lariat KR/6.7/CC/LB/FX4
2013 Jayco Premier 321 RLTS w/Atwood Levelegs (now removed)