It would affect the center of gravity some, raising it probably a couple or a few inches. Whether or not the roof structure would be safely able to carry the extra weight is a bigger concern; if it's a well-constructed RV, it ought to be able to, as it isn't enormously more than, say, a couple of air conditioners (the weight of which is less evenly distributed), but we all know that many RVs are not especially well-constructed.
If you do install it, be sure the tank is oriented lenghtwise rather than across the RV. If it's crosswise and half full, the water will slosh back and forth from side to side and could definitely cause handling issues or at least annoyances. Weight shift front to back is rather less of a problem--though still may be noticeable. Weight shift can, of course, be avoided by travelling with the tank either full or empty (as opposed to something in between).
All in all, I think a trailer mounted tank is a better idea (a "water buffalo" or something similar/smaller).
