Jerry,
Marty and Joel are right in that you can only add water or anything else to the fresh water tank on a 98 Marquis through the city water hose inlet. The tank is above the water inlet, so you have to fill it with a pressurized source because there is no access to the top of the tank.
I would do like Marty and pour bleach into an empty hose to get it into the fresh water tank, although Joel's idea sounds very good, I have never used it. Since I am a fulltimer, my water tank stays "fresh" from constant use.
Gerald