Sunday morning will be lighter on traffic and pedestrians that a weekday morning. Late at night is a good time to visit as well. Nothing like Times Square or the Empire State Building all lit up at night. Even though I avoid the place it certainly is a sight to see at night.
I don't know about the cabover but I would certainly go there with the smallest vehicle configuration you can. Coming out of the tunnels is sometimes a tight squeeze with the way the traffic flow is setup, especially if there is a moderate amount of other vehicles.
Good luck and "May the force be with you!" I avoid "The City" at all costs.