I use a portable waste holder such as the ones found here.
The one I have is only 10 gal so it fits nicely in the back of the tow with my golf clubs. When full I just leave it under the trailer until I head off someplace that I can dump it.
I actually dont need to use it all that often - the grey water tank will take 2 refills of the fresh water tank before it is full. Its easy to get by for 4 to 5 days without having to drain off the grey water tank if I dont have 2 people using the shower in the trailer every day. I have actually never had to drain off the black water as most of the areas where I dry camp have some sort of bathroom facilities to use during the day, saving the trailer bathroom for night use only, but if needed it could be used for that as well.