B2300 How much is too much?

I want to get a new B2300 and put a shell topper on it. I want the topper to be no higher than the cab of the truck. Does it have enough power to handle that? And what gas mileage would you expect from it? (it is rated at 24 city and 29 hwy) Now on top of that I want to occassionally put a 60 lbs kayak on top of the camper and run down the raod at 70 mph. What impact will that have on mileage?

Thanks,
Russell