I keep a "high pressure" hose wand, a couple telescoping long handle poles - with a three attachments (one brush, one "fabric blade", and one "water blade") - along with a pail - in the basement. When the windshield needs a cleaning while we're on the road - I wet the windshield with the hose wand, mix up a very mild cleaning solution in the pail - then stick the "fabric blade" on one pole and the "water blade" on the other and have at it. The combination of "fabric blade" / cleaner get the bugs and road grime off pretty well, the hose wand does a decent job of rinsing ... and the water blade kills the streaks. Using the hose wand lets you really limit the amount of water required - to the point that I've never run afoul of the "no RV washing" rules most campgrounds enforce.
The brush attachments along with the bottle of cleaning stuff all tuck neatly in the pail for storage, while the two long handle poles and the hose wand fit alongside the sliding storage trays so it doesn't take up hardly any space in the basement.
Every three or four months - I get a ladder and give the windshield a good hand cleaning followed by an application of RainX.
Combined it seems to do the trick.