Grey water? How's the ground in the area? Are you right in the sand, or are you in a heavily forested area? If you are in an area where the ground is highly pourous, and has a large amount of plant life AND you are not real close to your neighbors, you could run your shower water out on the ground, but only as you make it, not draining the entirity of the tank on the ground that has already accumulated.
You would of course, need the right kind of dump valve cap, and an assigned garden hose for the job. And be certain to drain it where its not going to wind up in an area where someone would normally set up camp. Usually I route my line into a dense copse of bushes or trees so that the water is coming out directly on their roots (50' of yellow garden hose).
Be certain not to flush any large food particulate out this method, unless you are going to dig a leech pit so as to bury any thing left behind, or scoop it out before you leave.
I use a 3' hose section where I currently reside in my camper to handle the grey water. The grass along side has grown like crazy and the chickens roaming the area will eat anything that comes out that hose (food wise) that I miss by accident before I can get outside to clean it up!