How many gallons do you think a person would need for a tank to wash down the brakes and some extra cleaning. Say you launch the boat flush till you park and a bit, go fish,then put the trailer back in and put the boat on and flush again and maybe wash what ever. I figure those fire extinguishers are 2-2 1/2 gallons X 2, so about 5 gallons. I'm thinking about building a similar system using a plastic ATV spray tank their cheap( $40-$100) plumb the trailer and have a hose with one of those SurFlo wash down pumps. Wire the whole thing into the trailer plug for power. I'm thinking about a 10-20 gallon tank (to much ,not enough) just in the planing stage.