I have recently started using a foam canon on the power washer.
DUSICHIN SFL-001 Pressure Washer Jet Wash 1/4" Quick Release Adjustable Snow Foam Lance, Foam Cannon
https://www.amazon.com/dp/B00WPKHFA6..._EtrHybSWM9PVZ
The device applies thick foam (RV soap of your choice) coating on the trailer that I let sit for a few min to soften up the dirt before using the soft bristled brush. I then rinse with the power washer.
Best method for washing I have found so far.
I would not recommend using it in the roof or awning.
I use RV soap and a soft bristled brush on the roof 2x yr.