Don't know what type of rig you have but if you hook up a hose to fill the tank just pour a cup of bleach into the hose while attached to the rig then hook up to the hose bib and turn the park water on and fill your tank.
You have to fill the whole tank up so the bleach does its job and drain the next day and flush again with water to try and get the smell out.
I use a short length of fresh water hose about 3 feet long with garden hose shut-offs at both ends. I attach the closed male end to the fresh water fill inlet. I hold up the female end and fill the hose with chlorine and close the shutoff. I connect the female end to my fresh water hose that is connected to my house outside hose bib and turn the water on at the house. Open the garden hose shutoffs and the chlorine will go into the fresh water tank under pressure. Repeat if needed depending on the size of your fw tank. The garden hose shotoffs help with spilling to keep the chlorine in the short hose until you connect to your house.