Smelly water after 3 days

I agree, most likely bacteria. you need to drain completely, flush completely and then fill and sanitize. Bacteria isn't a good thing to mess with. If you are not sure how to accomplish this, get with your local RV shop. You can get enough advice, suggestions and such form here, but we all have our own methods and for most they all work fine and are safe, but you need to be sure for you. Here is mine: we keep fresh water in our tanks all the time, this is from city water that is treated and has chlorine. This way bacteria doesn't or at least shouldn't grow. If we don't use our unit at least once per month, the tank get drained and refilled, and of course all the components get flushed thoroughly. We use a 10% bleach water solution, and make sure we clean our hose, spigot etc. whenever we hook up anywhere, including at home. I also when filling while on the road, make sure I put 1/4 of the bottle of bleach solution, which is about a pint into the fill hose which ends up in the tank. This keeps the fill hose clean and also makes sure I get the little bit of extra chlorine in the tank. In over 15 years we have never had an issue with smelly or bad water and never got sick. The only issue we had once was a bit too much chlorine and we smelled a bit of it, but soon was gone. Be careful of leaving the tank empty as bacteria grows with air and warm temps as well. Other ideas, suggestions and process are out there, follow what you need to to get the job done. Good Luck & Be Safe ! ! !