Like some here, I've been RVing for many years. To this day, I've never had a need to flush the lines or the tank on any of our rigs. I don't do it with the home's plumbing so I certainly don't see a need to do it with the camper.
On this note,, We got behind a slow poke at the dump station the other day. He dumped one tank, then hooked up the non potable water hose and flushed the tank (I'm guessing), then moved the slinky to another point on the trailer and dumped some more. We sat there watching for a good 15 or 20 minutes before he finally pulled up enough for us to dump. Took a little over a minute for me to dump our tanks, button everything up, and wash my hands. The guy was still fiddling around with his plumbing when we drove past him.