My current solution:
Clean the camera lens area well, apply a light coating of Rain-x Original. I made sure I had had not left any streaks or hazing, by buffing well.
This was moderately successful, but grim still tended to collect on the lens.
I purchased a new empty spray bottle and filled it with tap water. I park in a heated garage so I was able to place the spray bottle for easy access when parked. When I get home from a grimy drive, I grab the bottle and give the lens area 3 - 5 squirts. I have not had need to physically touch the lens, decreasing the risk of scratching. The picture remains clear when the lens is dry. I have only been using this techinique for about 3 months so can not speak to the long term reliability of it. I suspect that there is some risk to the camera/lens from being subject to "mini power-wash" on a regular bases, but so far the benefits of clear view have outweighed my anxiety of potentially having to replace the unit.
On the few occasions that I did not spray off the grime immediately on arriving home, I have used a few extra sprays with good results, but spraying clean while still wet has shown the best results.