I usually dump it as I roll down the road, there isn't usually very much so it's pretty much a non-event. I'm sure a LEO is going to hop right to it if someone calls and complains that an RV is losing water as he moves down the highway. By the time he gets to me, if he ever does, I'm not draining anymore water. If he has evidence that I'm draining waste water on the highway, more power to him. Since I'm not, no biggie.
People simply worry too much.
I worry more about those stock trucks that are "leaking" as they roll down the road. I'm pretty sure they aren't dumping fresh water. :)