Last year I decided to do the Dawn and borax flush on the gray and black tanks. After about 100 miles of sloshing around in the tanks, I dumped them right after I got home and closed the knife valves. About a month later, we took the camper out for a week. When we got home, I was dumping the tanks and found that the knife valves on both tanks were stuck shut (the first time this has happened in 10 years). I was finally able to get them opened, dumped and flushed. I was able to get them lubed and now they are working properly.
I would never do the Dawn and borax flush unless you have a way of lubricating the rubber seals on the knife valves.
I drilled a tiny hole in the end of the valve where the rod the handle is attached to. I sprayed silicone lube in and put a small screw to seal the hole.