I agree with Dusty using mild dish soap and hot water. Take your time and don't try and scrape it off! Probably going to take a few times, but I bet it will come clean.
I didn't look at my roof too well either but it's not too bad. It has two patches but no leaks anywhere which is the most important thing. Roofs can be repaired. Plus I got my for a great deal too! just like you!
Any leaks that you noticed inside?
1992 Ford F-350 XLT, Crew Cab, Longbed, 4X2, 7.5L, E40D, 3.55, Tru-Cool Max Trans cooler
1998 Jayco Eagle 243RKS
Anderson Ultimate 5th Wheel Hitch
2014 Gregor Eagle Center Console, Mercury 60HP 4 Stroke, Garmin 93SV FF, Motorguide Xi5 GPS