Washed the Montana yesterday and instead of using a car wash product, I gave Murrhy's Oil Soap a go at the exterior. I've been using Murrhy for years, around the house and even my car's interiors.
So I diluted following the instructions, quarter cup of Murrhy to one gallon of water. It worked really well, bug guts came off the front cap with a little scrubbing and black streaks came off or were greatly diminished. After washing I did hose down the sides, the instructions differ.
When it dried, it looked as if I just finished waxing it.
Solute to the OP.