I have a 6 Gallon Tote-N-Store
I personally would not go any bigger as others have said the bigger the tank the heavier it is and the more problematic it is to find a place to store it & dump it & the more reluctant I would be to actually using it. Mine is small enough that if your at a place with no dump station you can take it elsewhere (a gas station bathroom for example) and carefully dump it down a bathroom toilet & flush it away - obviously not all in one pour! LOL
I use mine mainly for draining off grey water only, as the places I camp they usually have an outhouse so I can avoid the filling of the black tank as well. Cleaning it out after grey water only use is easy - once home fill with fresh water and put a bit of bleach in it and let it sit for awhile. Keeps it from getting stinky and you can be stored in the back of the vehicle or in the trailer without an issues. Someone recently told me that dropping a good denture cleaner into it and leaving it sit with water is an even better way to clean it (and the blank tank in the trailer as well). Have not tried it yet!
I use the short garden hose connected to the grey water discharge pipe that has a cap with a garden hose connection on it. Makes it real easy to control the flow of the water & not having to deal with the larger slinky type hose.
Lots of ways as others have suggested to keep water usage down. Having a shut off on the shower handle is one but the one I think saves me the most water is the use of a tub in the kitchen sink when doing dishes. Especially when it comes to rinsing off the dishes. Having the water run into a tub that fits in the sink rather than down the sink pipe is a real good reminder as to how much water you are using. You can then dump the tub water into the black tank that more often than not has the room for it.
A tote with wheels is good to have & on occasion wished mine had wheels but the wheels are only usable if where you are staying actually has a dump facility and paved road that you can pull it along. Most of the places I find I need to use the tote more often and not do not have a dump facility and or a paved road.
I found with the Scamp
I could easily get by being careful with water usage & 2 people having showers for 4 days - longer if I am traveling on my own - before needing to drain off some grey water. So the frequency that my tote is actually used would not warrant one of a larger size and giving up of storage space for it.
If you do a google of this site you should be able to find a post where someone rigged up a holder under their Scamp
? using PVC pipes for storing their small portable tank.