We started with a 25ltr bucket but found it too big to get out of the side hatch in the camper....so down sized to (around) a 15 ltr one which seems to work well for us. We store the washing detergent/fabric softener/pegs/ and a container of coins for washing machines when and if we stay in a park somewhere in ours. The bucket also just has a push on lid and hasn't leaked yet as we washed on the go across some pretty rough terrain. Recycled builders products buckets do seem to be the go and cheap as if you can find a building site with some available!
Cheers,