I would not recommend Dawn or any other dish detergent for any vehicle (car,trailer, or boat) There are some rather harsh ingredients in dish detergent in order to get off baked on food. Your finish will be much better served with an automotive soap available from any auto parts store. It will be much easier on the coating and will work for the roof as well.
I personally use Griot's products on everything. (Not that they are the best,nor is this an endorsement. It's just my personal preference.) That includes my collector cars as well as the Airstream.
Craig and Carol
1997 34' Excella 1000
1970 27' Overlander, International
2009 Ford F150 5.4L
ProPride hitch with 1400# bars
Past President Southeastern Camping Unit (12)