You don't need to fill it up!
The stuff, whatever it is, is on the walls, not in the space in the middle. Therefore, you only need to wash the walls.
Take it out of the van and into your garden where you can manipulate it.
Chuck in a bucket full of really hot water and slosh it about to wash all the walls including the top. Repeat this several times. You might need ten or twenty buckets full of water - about the same as you'd use in a deep, luxuriant bath!
This is probably faster as you can be sloshing one bucketful around whilst the next bucket is filling from the tap. You'll get the entire job done in less time than it requires to just fill the tank once!