This is the method we found best when travelling around Australia in drought conditions and using a hose to wash a vehicle of any size, was definitely something you couldn't do.
Buy a container of CT18 truck wash from Supercheap or Repco, plus a good vehicle wash brush with a long telescopic handle and a good bucket. With these 3 items I can wash our 7.3 metre in length and over 3.5m high, cabover motorhome with just two or three buckets of water. I use the same method to wash the Suzuki toad as well.
First bucket, water and CT18, go by the instructions on the bottle as to the amount of the product for the size of the bucket. Apply all over, then leave as long as you like, if really dirty, leaving overnight I find is fine.
Then use one or two buckets of clean water and your clean brush to wash off the grime and the first application.
I find using this method I can wash the motorhome almost anywhere, roadside laybys, free camps, POPs if you ask permission. CT18 has nothing toxic in it but pays to just watch that you are not too close to a waterway for dirty water run off to affect the waterway. Every year or four I give the motorhome a good polish, due for one now.
Note that the other CT product, CT20 cannot be left on the vehicle, but CT18 can.
I still use this method today, even though water is more available here in NZ.
Hope this idea helps,