I don't believe you can buy a TM from the factory - they sell only to dealers, and don't do retail transactions.
Be aware that in the rest of the RV world, it is not uncommon to find that a busy dealer service shop will not schedule you for repair or maintenance service on a unit you did not buy from them. They are just too busy. Or they may move you to the back of the line. I've never experienced this on either of my two past TM's, and I don't know how this plays out in the event you bought a used unit from a third party. I do know that some dealers (I'm thinking of MCD on Cape Cod in particular) do not subscribe to this theory, so perhaps TMs are different. But you might give a bit of extra consideration to the dealer nearest you.
You might take a look at this thread, where the same basic question was asked.
https://www.trailmanorowners.com/for...ad.php?t=20104
Bill