Hmmm - we'll leave Ben out of the discussion for now, but the marginalisation of Marx I would date to roughly the post-Alice/Anaconda period - there's an interview on here where he discusses his dissatisfaction with the production on Anaconda (which Von - having reckoned he had learned all he need know about studio duties - took on himself).
Ah, here we go:
http://myheartland.co.uk/viewtopic.php?t=2140
And check out here as well:
http://www.side-line.com/interviews_com ... 0_0_16_0_C
All the RH tracks are officially Von solo compositions, and with Von's, I would guess, determination for a repeat of self-production, it seems reasonable to suppose that after the Anaconda experience Marx just opted out. Something like "yeah, whatever, do it, I don't care".
All speculation, of course, but it fits the facts we know without getting too far removed from reality.
Rumour, by the way, has it that a certain Mr Adams was also involved in RH, and that he may deserve a co-write credit on ToL.
In both cases we'll probably never know the full truth.