I work for a restoration company which means we clean up after fires, floods, broken pipes, and other disasters. In many cases, we have to extract water or sewage from buildings using a truck mounted extraction unit and then dump this water somewhere. The closest place to our shop is a Flying J, and I have a list of RV parks and other locations throughout the city for us to dump as convienently as possible.
If you own or operate a park and you have a dump station, you might want to look in your phone book under "Water Damage Restoration", "Carpet Repair" and "Carpet Cleaning". Write to these companies and let them know you can offer your dump station for a fee (maybe a discount) and your location. Remind them that dumping contaminated water anywhere else could bring stiff EPA fines!
Just a suggestion, hope it helps.