I've been using an RV brush on all my campers, since we started with our pop-up. I still have the original brush. Camco RV brushes are soft, designed for RV and work well on all surfaces.
I since purchased a second one, with a retractable handle that also attaches to a garden hose, so it can shoot water non stop (works great for rinsing).
For my original brush, I just used a long painters handle that extended out 15 feet. It works great for brushing leaves and twigs off the top of my slide out too.
These brushes are the only thing I wash my vehicles swith, and I even use them on the vinyl siding on my house, and those plastic shutters too. So, they are safe for any surface, soft, won't scratch, and probably the best you can get as they are so versitle.
You can pick one up anywhere. Here's Wall Mart's version:
Click here.If you want the brush only, and get your own pole, wand, or stick, like a painter's pole, here's Camping World version of the brush only:
Camping World click here.