Our experience is that towing a vehicle (dolly or flat - we did both) behind our motorhome reduced our mpg from 8.5 to 7.5. We had a 28' Class C "widebody" with the E-450 chassis. The dolly/car offered no wind resistance.
We recently towed a Pruis C home from FL for our daughter, behind our Transit T-250. The tow dolly was wider than the van, and dinged us about 4 mpg which we suspect was due mostly to wind resistance.
Regarding the ease of dolly towing vs flat-towing? We just made a list of each step to take with either method. The hassle of moving around the dolly once camped, is offset by the ease of bringing almost any car along as your toad.