It seems a little low to me but not by much.
I have a 1989 Dodge B250 with a similar top. I did take the rooftop A/C off this past winter as I found it was really catching the wind badly in a storm but so far mpg is about the same as last year.
It'll get anywhere from 10 1/2 to 13 mpg in mixed driving as I use it quite regularly as a daily driver. I know the lowest was during the winter at as low as 7 mpg in January but can't count that as it's irrelevant to your driving.
So far this summer it's been getting 10 1/2 to 11 1/2 and though a lot of in town we are in a small village in a rural area so fairly long runs. I do fine that dropping the cruise from about 100 km/h (62 mph) to the 85 km/h (just about 50 mph) will make a 2 mpg difference. Tanks where I've done a lot of back roads and steady at the slower speed will return closer to 13 mpg although I only fill up about once a month so still getting a handle on this summer's mpg.
This may not help a lot but since it is real world driving and I keep track of mpg (U.S.) to the 100th of an mpg I thought I'd throw it on here.
Cheers.