Spots On My Camper

[TyeeMan]

Over the summer we keep our camper on our land in northern Minnesota. Last year I got a few of these spots (pictured below) and last year I got peppered with them. I don't know if it's fly crap, or sap from a tree. The spots are only on the rear and the street side, the two sides that get the most sun during the day.
Darn things are super resilient! I have yet to find any kind of detergent that will even touch the things. So far the best thing I've found to remove the spots is super hot water and my thumb nail. At that rate I'll be removing spots for the better part of a day.
Has anyone come across these things? If so, how do you remove them, , , easily?

Hopefully the pictures will load.