Another possibility, I guess, is that the drain pipe was cut too long, and the top end sticks up out of the floor so far that the toilet actually rests on the pipe, instead of on the floor. This would cause the toilet to rock, of course.
What year is your TM?
I'm trying to see a difference between these two photos, each showing the top of a drain pipe. One is your photo. The other is from my 2006 TM, taken from the post linked near the top of this thread.
I'm thinking - see if you agree - that the top of the drainpipe is considerably higher in your photo than in the other. You say that your toilet always rocked, right from the beginning - so the overly long pipe could have been a construction error. You also say that the pipe won't push down at all. This could be because the drain pipe is anchored in the nest of other pipes underneath the trailer, and the nest itself is firmly mounted.
And the fact that the toilet is sitting so high that the legs won't reach the floor would explain the need for shims.
I'm running out of ideas here. Over to you.
Bill
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