With three kids, and several beach trips in the rig over the years, I no doubt have dumped a rather large amount of sand through the grey tank without a problem.
The beach sand around here is pretty hard to get off with that outdoor shower. even with soap, it's hard to get it all with the inside shower. Kids can collect a lot of sand on their bodies.
I figure most works it's way out over time, but I figure there's always some sand sediment in the bottom of the tank that just never makes it out.