There is no longer RTM in Windows 10. What most people understand as RTM though is whatever is the first build officially released by Microsoft as retail, MVS (MSDN) and/or VLSC. In the case of 1903, I see RTM as .30 build. However other people's opinions may vary.
I was under the impression that the WU version is 18362.30. Or is that the ESD version vs the UUP which is 18362.1?
WU downloads the UUP file set 18362.1 + SSU/CU/Flash/DotnetCU, the ESD now used by MCT/UA is 18362.30 (pre-integrated SSU/CU).
Added MSU links: https://forums.mydigitallife.net/threads/windows-10-hotfix-repository.57050/page-405#post-1526432
Afaik, that's part of the new policy, only people who really seek the update, by pressing on search for updates, will see new updates, if msft thinks you really need it, they will push it to you.
I checked WSUS. I think there are 2 kinds of updates. UUP which "may" be 18362.1 (I don't know in fact, so I have to trust you ) . ESD which is like here https://forums.mydigitallife.net/threads/windows-10-esd-repository.59082/#post-992532 , which is 18632.30 The ESD install is the "full" install/upgrade, similar to using an ISO while UUP is incremental (can be full too if required), something which was discussed many times before. So there are 2 releases for the update on WU which makes it all even more confusing.