fuel mileage better w/ a shell pulling a TT? If so, how much?

[speedracer]

Anybody have experience as to whether having a camper shell helps fuel mileage when pulling a TT? If so, what kind of improvement?
Thanks.

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'01 2500 QC LB 2WD 6sp HO SLT+ leather 3. 54 ltd slp BLACK trailer tow and the all important cab lights. (for hauling motorcycles)

95 Freightliner FLD132 70" Condo 470 Detroit 13spd 3. 36 (to pay for motorcycles)
89 H-D FXRS, 99 Triumph 955i, 86 Suzuki GSXR750 ltd, 00 Aprillia SL1000

98 Yamaha YZF-R1(race), 95 Kawasaki ZX7(race)