Marc Miller wrote most of the original CT rules that were compiled to form MT (which is why he is credited as sole author on the covers). Frank Chadwick, Loren Wiseman, John Harshman, Timothy Brown and the brothers Keith also contributed material to CT that found its way into MT. Very little of the text of MT was actually new material -- even most of the flavor text, examples, and introductions were word-for-word cut&pastes from existing CT books.
In statements at the time (from
Challenge and
The Travellers' Digest) Marc Miller is said to have been a big fan of the DGP task system and to have had lots of brainstorming discussions about it with the DGP crew. I believe MWM is credited with creating the "Hasty" and "Cautious" mechanics and the Uncertain tasks procedure. Note that MWM also adapted an earlier incarnation of the DGP task system into
Traveller:2300 in 1986, a year before MT, and it was purportedly
Marc Miller's idea to do a complete revision of the CT system integrating the task system into all the key mechanics.
Per Loren Wiseman (on either the TML or JTAS, I don't have the exact reference handy) the Rebellion situation was created and the broad strokes outlined by Marc Miller, Frank Chadwick and Timothy Brown. The exact details were then filled in largely by DGP and the HIWG. Loren himself was heading the
Twilight:2000 line at the time and apparently had very little involvement with MT (though I think he may take credit for having come up with "the Real Strephon").
Although DGP did the compilation and editing of MT, it was actually produced by GDW (layout and typesetting are credited to GDW's in-house team -- and look much more professional than DGP's efforts of the time (i.e.
Grand Census,
101 Robots)). Marc Miller chose DGP for the job and gave them his full and complete trust and the finished project received his blessing (and wouldn't have been published if it hadn't). He doesn't seem to have had much hands-on involvement in the actual development, but that was apparently by his own deliberate choice.
In the TNE era Dave Nilsen (and possibly Frank Chadwick and/or Loren Wiseman) bemoaned allowing DGP too much influence over MT and wished they'd kept more in-house control but AFAIK Marc Miller has never made any similar statements (at least publicly), and, in fact, at the Traveller seminar at GenCon in 1990 (the height of MT) proudly proclaimed that Traveller was "safe" in DGP's hands (by way of explanation for why GDW was providing so little support for MT and devoting most of their resources to the likes of
Space:1889 and
Cadillacs & Dinosaurs).
If anyone (especially any of the "Ancients") wants to dispute or correct any of this, please do. I'm not and have never been an insider and claim no more authority or knowledge in this manner than any other longtime fan (and amateur Kremlin-watcher
).