Portable gray water totes are pretty flexible for their use and come in many sizes. I have one when we dry dry camp because I carry an additional 84 gallons of fresh water in the enclosed trailer I will transfer to the TC when its tank gets low. We try to shower outside to limit gray water storage, but when full I will dump the gray tank into a tote so we can continue camping without having to find a dump station. Our tote has a hitch accessory that allows it to be pulled behind the TC - I would not suggest this if you are going over rough terrain or any extended distance, but it would work well in a campground. I will roll mine into the enclosed trailer and secure it until we get to a dump station.
BTW: I would rather deal with a portable gray water tote than black water...