Roof downspouts always seem to leave black streaks along the trailer side walls because the spouts are too short. After trying metal downspout extensions and small PVC tubing, which all eventually came off, I found the simplest and most effective solution — clip on a clothespin at each downspout.
The water wets the wood and runs off at the end of the clothespin, well away from the side wall of the trailer. The clothespin trick works to control runoff from morning dew drips and major downpours.
Remove the clothespins when traveling.
For additional handy tech tips from RVers just like you, order Ten Minute Tech, Volume 3 in the Good Sam Club Store.