Ok, so are you using the fresh water tank or are you hooked up to the water at the campground?
If it's the tank, then it's a good probability you need to sanitize the tanks. Just like when you dewinterize. Herk's got a good step by step on this process in the FAQs (Here
), but basically it's add some bleach (like 1/4 cup per 15 gallons of tank) fill the tank with water, run all the taps until you smell bleach, let it sit for 24 hours and then drain it all out.
If it's the campground supply, well sometimes they're on well water there and it could be coming right from the supply. Try running a tap at the hookup and see if there's any smell there. Might be due to water sitting in their own lines, deteriorating plumbing in the campground, etc... In this case there's not much you can do other than running the tap before hooking up and making sure you're using a filter. I ran into this at one campground during their opening weekend. I ended up using my onboard fresh water for every else and only hooked up to the campground's supply to take showers.