The best thing to do when this happens is to sanitize the entire water system. First drain all of the water out of the system, and then close all of the drains.
Take ? cup of household bleach for every 15 gallons of water that your fresh water holding tank holds. Mix the bleach in one gallon of water and pour it into the fresh water tank. Fill the fresh water tank with potable water. Turn the water pump on and open all faucets, one at a time, and run water until you smell the bleach. If possible drive or tow the RV to assist in cleaning the entire holding tank. Let it sit for twelve hours and then drain the entire system. Refill the fresh water holding tank and run water through the system until you no longer smell the bleach. It may be necessary to repeat this more than once. When you no longer smell bleach the system is sanitized.
You may want to consider filtering your water before filling your holding tanks by using two "whole house" filters commonly used in household applications. The first filter is used to remove sand, rust and larger particles, while a second charcoal filter removes taste and odor.
Start by sanitizing your system in case the water has become contaminated, then if you still have sulfur odor, check the source of your potable water. IAC filtration is your best overall protection for your water system, and replace the filters on a regular basis.