I did mine about a week or so ago. I just squirted some in there and filled up the bucket until I thought it was full enough. Probably about 3/4 full of water. It doesn't take much dawn to make for some good soapy action.
My roof was actually pretty clean. To be 100% honest, this is the second time I've washed the roof and I really didn't see much difference in before and after either time.
One would think washing the roof is a pain. Compared to the awning, its a breeze. If you can, when you put away your trailer for the year try and get the awning as clean and dry and possible prior to. It will make your life easier the next year.