Although this doesn't respond specifically to Twain's influence on
modern writers, it is an interesting bit of trivia.  I once sent a letter
to Larry McMurtry, a copy of a letter that Twain wrote actually, in which
he apologized to the county clerk of Archer County Texas for late payment
on the taxes of land he owned there.  As I remember, Twain said that if
he ever ran into the agent who was supposed to pay the taxes on his
behalf, that agent would "beg for brimstone, and he would beg in vain."
I sent it to McMurtry to show him that he was not the only famous author
to have owned land in Archer County.

     McMurtry wrote back and said that he was glad to know it, and that
he would check to see if they owned some of the same land.  I guess they
didn't since he never wrote back.  But I found it an interesting
fact, nonetheless.

    What about Kurt Vonnegut?  I haven't seen anyone mention his works.
His essays, particularly, have been influenced by Twain's wit.