Towed my current van with a Falcon Ute. Then towed same van, same ute but with a canopy. If anything, the airflow over the canopy caused more drag that without the canopy. Will never know now as I sold the ute last week. About to head off with a new Ranger Crew Cab, no canopy, but same van.
Philbe - I'd imagine any difference canopy/no canopy would pale to insignificance compared to the wind drag of the caravan.
Good Luck.
ps...don't be shy to ask questions which is what the forum is about.