I just saw your post, glad you were able to locate the bypass valves. We have a 2020 29M also. We bought it in November so the dealer winterized ours for us before we took it. I watched the tech go through the process so I am familiar with it. For your information, as I was going over the coach after we got it home I discovered there was a macerator pump under the sink in the outside kitchen. This pumps the drain water UP into the gray tank. The tech apparently did not know this because all he did was pour some anti-freeze into the sink but did not pump it through the system. After looking over the plumbing under the sink I saw that you need to run that pump long enough to fill the 1-1/2" PVC drain line with antifreeze to protect it. It looks like it could hold a couple of quarts of water/antifreeze.
BTW, the pump switch is located under the cabinet.
I like to travel, but I also like to get back home.