If you get into headwinds and your camper is high (8-12 ft) a cab high top is the way to go. Because the wind is broken ahead of the trailer (actually the trailer is drafting behind the pickup) so you have less square ft of surface area on the front of the trailer for the wind to hit and take more fuel to pull the trailer thru the wind.

 

I just ordered one of the Leer 122 caps. It is a high back and should help block the wind for a bulky trailer. Bikes and quads can be loaded in. Here is a pic:
Cost me about $1500 installed with lights and boot.
http://www.leer.com/fordgallery/f150extcab122.jpg

I have seen people bolt spoilers on the top of their caps just like the big rigs and it probably helps a lot pulling the flat-nosed trailers.

 

MIne was about $1350.00.
So far it has had no problems,
they painted it to match My blue lemon.
It realy does look good on the truck.
I'll probably post a pic or 2 in the next couple of days

 

I have an ARE Z series cap on my long bed CC and did not see any perceptible change in mileage (Z type is even and flat with the roof of the cab). I have 3 trailers that I use to haul the big tall stuff. I use the volume and partial security of the cap all the time. Understand the utility of the open bed, however for my use, the cap is the best answer. Caps make it harder to navigate for parking, but I use my mirrors and combat park (i.e. back in).

 

If i can find the link I'll send it but I have read a very detailed academic aerodynamic drag study (the focus of the study was to address the myth of improving mileage by dropping the gate) that concluded that dropping or removing the tail gate on pickups reduces fuel economy but only by a small amount (like .3-.4 miles per gallon if memory serves me right). This was a very impressive physics study with tremendous data. While these researchers did not test bed caps they also concluded that caps would most likely reduce mileage. The conclusions are kind of counter intuitive but relate to the air bubble that KW mentions above. Essentially the tail gate encourages a circulating air bubble in the truck bed that results in less turbulent airflow over the back of the truck. In the absence of this "circulating air bubble" effect, aerodynamics around the sharp transitions such as the vertical decline behind the passenger cab or bed cap are characterized by a large amount of turbulance. Essentially this results in increased "backward pull" on the truck via increased drag diminishing fuel economy. think about the shape of the back of a time trial bike racers helmet. That odd extension off the back reduces separation of the flow= less trubulance=faster biker. However, to more directly address the point of a cap in conjunction with a square camper tow, I can also see (guess) that the cap could reduce the wind force on the forward face of the camper and result in a net gain in mileage versus towing a camper with a truck that had no cap (despite the ill effects of the above mentioned increased drag on the truck itself).