I ended up painting the roof with regular white acrylic enamel automotive paint. 1 gallon ($35) with hardener and slow reducer sprayed on in one heavy coat with probably 16 oz left over. It has made a fantastic difference, especially when combined with the reflectix window shades.
My build thread:http://www.skoolie.net/forums/f11/build-thread-for-haulin-oats-and-goats-11237.html#post113500
A gal, a guy, three cats, two dogs, one rabbit, and one goat, traveling the country together.