I've been around Airstreams restoring them since 2003... The roof coating thing has been discussed hundreds of times and only one product name has stuck with me over the years, 'Super-therm'.
A long-distance buddy of mine ran a campground in Canada and wintered in South Florida and jumped through all the hoops to get Super-Therm and described it as ultra white whipped marshmallow creme. One of the key designs is to reflect ultraviolet, visual light and infrared energy, and it is an industrial coating made to weather pollution and all that rot. It is applied to bus roofs, trucks, trailers etc..
He was very happy with the choice - also, he used a different HSC-1000 insulating paint as an interior coating on the vertical sheet metal walls to protect the 0.032" 2024 aircraft aluminum as base coat to protect it from commercial spray foams' heat and heave action, that's buried around post 540 in his thread. Everything was perfect until a 4' wet snowfall collapsed the shell across where the door opening weakened the Airstreams eggshell strength, he collected the insurance and parted off the wreck.
There may be a hitch is getting it in small quantities, one sale for a 8-10 mil thickness coating for 70sqft does not interest their commercial sales people very much.
Least cheesy info sheet I could find:
http://www.eaglecoatings.net/content/supertherm/pdfs/SUPER_THERM_vs_WHITE_%20PAINT.pdf
The US distributor:
http://www.spicoatings.com/products_solutions/super_therm
Link to the Airstream project:
http://www.airforums.com/forums/f537/shell-off-headroom-epiphany-79254-25.html#post1081700
Again, haven't used it myself but I've only heard good things about it.