There are two possible sources of sulfur smell in your water. One is a high level of sulfur in the water. When that happens you have sulfur in both the hot and cold lines.
The solution is flush, flush, flush.
The more common source of sulfur smell is when it is in your hot water only. In this case natural anaerobic bacteria have gotten established in the tank. This usually only happens when the rig is left for a time, and all the O2 in the water is used up (O2 kills anaerobic).
The solution is flush, flush, flush. Although some bleach in the first flush would not be a bad idea.