First light when dew is on is the best time to attack them.
Spray with water to loosen even more. Then wash with soft brush and no streak car wash. Use the brush as much as needed. It's not hard when they are soft.
If they are hard park in the shade and cool and soak with water and wash with plenty of suds and brush.
It's aggravating, to have to get up first morning, in camp early but it is the best time to wash the windshield and rig. If they won't let you wash rig they usually have no objection to windshield.
A bucket of suds on brush to soften first them more to wash and then rinse by throwing water on it or whatever you have to do.