We were just there last February and throughly enjoyed our visit! Parking with an RV could be challenging but if you're willing to walk a little you "may" be able to park on one of the side streets around the corner where you won't need to angle park. A lot is going to depend on what time of day and just how busy downtown Metroplolis is. While we were there, there was PLENTY of close parking (for an RV) BUT you're talking about fourth of July week and that's going to make things harder.
You might give the chamber of commerce a call and see if you can get info from those people. While there check out "Yesterday's Home Cookin" diner. It's a cute diner we found and had LOTS of locals there for lunch the day we were in there. The food was really good and the prices VERY reasonable!
Good luck and enjoy Superman!!! :B
Dan