What Ned says - rarely any need to wash or wax the top of a slideout.
As for washing the rig, don't be afraid to hose it down. While I wouldn't intentionally spray directly into a slide out seal with a pressure nozzle, the normal wash & rinse kind of activity is a non-problem. Or if it is, it is best to know about it and have appropriae repairs made before the next thunderstorm.
A side note on slide toppers: Some Safari & Beaver coaches of around 2000-2002 vintage had slide out roofs that were molded to provide water runoff and did not use toppers. The design worked well for water but only fair if there were leaves, branches, etc. on top. I believe this sort of slide roof is no longer offered..