I do have related, real world experience with the wind deflectors. I put one on the rear of my Leer shell on a F-150 SCAB. This got it close to the front of my 22' TT. Fivers get a similar benefit as they are so far forward in the bed with the deflector mounted on the cab. I would guess the typical fiver ends up being about as far from the deflector as my TT did.
Did it work? You betcha... had it on for 3+ years and consistently got 1.5 to 2 mph improvement over running with no deflector. While that is not a huge change it paid for the deflector in about one year. Another way you could tell it was sworking was the absence of bugs on the front of my TT - if they aren't hitting there neither is most of the wind. I only mounted it when towing {no resason to have them mounted if not towing} so no downside doing the daily driver thing.
BTDT and it worked {very well} for me.
As always... Opinions and YMMV
:C