My darkroom is also not hooked up to house plumbing. I have 50 gallons of storage and a large sink. RC papers don't need much, they go in a first 5 gal bucket with a little sulfite and a second 5 gal bucket for 10 minutes. 10 gal of water to wash up to 20 8x10 or equivalent. For fiber, I fill the sink. It is about 20 Gallons, I can set 11x14 on their edges completely submerged. After the first 5 gal bucket with sulfite for 6 to 10 minutes they go in the sink for 30 or more minutes. There is a lot of water in there and the dilution of Thiosulfate that remains in that wash water would have to be very low. I do up to 15 11x14 or equivalent that way in a printing session.