The best way to clean the Krystal Kote is with regular car wash soap. You really can't polish it, but you can wax it. The Krystal Kote is designed so you don't need to wax, but I have and it helps a little. If you don't wax the whole TT, you should at least wax the logos and striping, especially since Sunline isn't around anymore to get new logos.
If you are describing black streaks when you refer to cleaning, they are almost impossible to prevent. The best solution is to keep your roof as clean as possible. The roof is the source of black streaks, so keeping this clean will keep the rest of the TT clean.
2007 T-286SR Cherry/Granola, #6236, original owner, current mileage: 9467.8 (as of 5/26/19)
1997 T-2653 Blue Denim, #5471
1979 12 1/2' MC, Beige & Avocado, #4639
Past Sunlines: '97 T-2653 #5089, '94 T-2251, '86 T-1550, '94 T-2363, '98 T-270SR