I use the same Black streak remover. I wet the trailer down with water, spray the Black Streak remover directly on the trailer and use a brush to wash/scrub/rinse the trailer.
Works just fine with very little effort. I would suggest that you wax the area if you use my method. The Black streak remover also removes the wax.
I have a thread on here somewhere to show the difference. I will see if I can find it post for you.
HAPPY CAMPING!! :D
Daryl and Sandy
2013 Eagle 314BDS
2012 Ford F150 Platinum, 4 x 4 Crew Cab with Ecoboost