I always raised the roof so that the top was just at a comfortable reach, and then pulled the tenting out and let it hang to clean the top surface. Make sure all of the windows are fully zipped. I used a sponge and a soft brush for any tough spots. Once done with the roof surface, raise the roof the rest of the way, carefully extend the bunks, and setup the tenting to clean the tent sides and dry.
2011 Jayco X19H (purchased 2015)
2008 Jayco 1007 PUP (purchased new, traded for the X19)
2012 Chev Traverse LT with TP