To catch our gray water we use a plastic bucket.  Usually it's one of those shorter ones, maybe 2.5 gallons, that bulk laundry soap come in.  The kind with a snap on lid that you cut when you open it but it will still snap back down pretty secure.  You can buy those 'seat lids' too, of which I have a few.  I cut a hose hole into the original lid.  I have a short piece of hose that coils up in the bucket, or attaches to the bus/camper/trailer whatever drain and goes into the hole in the lid during use.  You also have an extra stool.
  Speaking of stools, in an emergency or if you are camping where there are no johns, you can use the bucket as a porto-potty.  I've found some kitchen bags that are the right size, which I use to line the bucket..Tie it off and dispose of it when you find a proper place..
  A versatile item, a bucket...or two or three, they nest pretty well and in the short version, mine will fit under the fold down bed/bench seat..
  Don Hanson