Origins Of The Trethowan Name 

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After much research I must conclude that the "Homestead by the Sandhills" is the closest meaning of Trethowan as source #8 suggests.

Source #1:
    From my aunt Diane:  The name comes from an old Celtic phrase meaning "Hearth of the Owens Clan."

Source #2:
    From the Cornish Cousins web site:  This is not for the full name, but rather a component of the name "Tre".  Tre is a place origin or to be in proximity or come from that place.

Source #3:
    From an e-mail sent to me from John Symonds:
... I have a couple of books about Cornish names and surnames which certainly have material which would be of interest to you.

Source #4 (I concider this the actual definition):
    From http://freepages.history.rootsweb.com/~kernow/names_t.htm:
    TRETHOWAN tre-(D)Thewin, homestead of Dewin, (d mutates to th)

Source #5:
    From misc. source:
    TRETHOWAN from the farm by the sandhills

Source #6:
    http://www.summerlands.com/crossroads/celticlanguage/labara6.html
    Divinour 'diviner, fortuneteller, wizard' - compare Welsh DEWIN, covered in "Labara 4: A Taste of Welsh Verse"

Source #7 The Welsh Lexicon:
    http://www.cs.cf.ac.uk/fun/welsh/LexiconWE.html
    dewin [-iaid, m.] - (n.) mage, diviner, magician, wizard, augur
    As for "Sandhills" closest is - bank of sand - (n.) traethell [-au, f.]
    As for Homestead - cartref [-i, -ydd, m.], pentref [-i, -ydd, m.], ty+ [tai, m.], tyddyn [-nod, -nau, m.]
    As for Owen - (prop.n.) Owain

Source #8 The Cornish Lexicon:
    http://www.pauldavies.net/cornish/lexicon.cfm

TRE, f.: town, plural; trevow.
    Sandhill - (m.) towan

From these differing sources it is safe to conclude that "Tre" can have the meanings
    From, By the, Of the, Home stead of, Hearth. . .This squares with Source #2.
    But Still there is no firm agreement on Thowan/Dewin. Owins, Sand Hills, Diviner or Wizard?
   There was a farm with the coat-of-arms presenting 3 shieves of wheat, hung above the door.

The first is the book "Cornish Names" by T.F.G. Dexter, published by D. Bradford Barton Ltd in Truro CON. First published in 1926, mine is a 1968 reprint. Here is his TRE definition:

TRE, f.: a homestead and its surrounding buildings, a "town" as the Cornishman understands it in "Church Town".  In Scotland "town" (O.E. tun) is still used for a farmstead and its buildings.  Tre is generally used in the singular, and is an abbreviation of TREV f.; p. trevow, trevon, treven .  The v is generally dropped before a consonant, and it is sometimes difficult to decide whether the v belongs to the trev or to the epithet.

The second book is "A Handbook of Cornish Surnames" by G. Pawley White, published by Dyllansow Truran in 1972. Mine is a second edition of January 1981.. Here is his statement about the surname Trethowan:

TRETHOWAN from tre-(D)thewin: Homestead of Dewin.  Place name Trethowan,  Constantine.  Spelt Trevewen 1295.  Found in Mid West Cornwall.

I must explain the (D) bit in this explanation. In Cornish, adjectives like nouns can be of two genders, masculine and feminine. The adjective which follows a Feminine noun is modified to agree with the Feminine noun. This modification takes place at the beginning of the word and not at the end.

In the name Trewthowan, we have the feminine noun Tre requiring that the epithet Dewin should have the D mutated to TH. Another example is the name Carthew, which could be translated as the camp of Dew or as we would say Dew's camp. There is the possibility in Pawley White's interpretation that the "in" on the end Dew is actually a plural of Dew. I have discovered that Dew also can mean "god" so Trethowan might mean the homestead of the gods!!!

Dexter gives an example that Saws means Saxon and Sawson means Saxons, so that Tresawsen means the town of the Saxons and not the town of the Saxon. Perhaps in our case, I should have written Dews' camp!  Trethowan  could become the homestead of the Dewins.

Source #4:
    From an e-mails sent to me from David Trethowan:
I LIVE IN Constantine where the Trethowan name seems to have originated Trethowan Manor(remains) is nearby.

The earliest records of the name Trethowan are detailed in a book called "Constantine in Cornwall "by Charles Henderson where he mentions that in the time of the Doomsday Book it was owned by the king (William The Conqueror ) . I quote:

"Trethowan (120 acres) gave name and origin to a family of small gentry, resident here in the 16th and 17th centuries. The name Trethowan is still found in the parish. Alan Trethouen paid the subsidy in 1327. Henry Trethowyn did homage to the Lord of Merthen for a Cornish Acre held in Trethowyn in Knight's service. In 1528 part of Trethowyn was held by the heirs of Trethowyn and part, with Polpry, by Gerveys of Bonallack, both under the manor of Merthen in Knights service.

John Trethowan of Trethowan married, in 1588, Avis, daughter of Thomas Enys of Enys
by Katherine Reskymer of Merthen. In 1649 and 1660 Thomas Trethowan was living at
Trethowan, which was his Freehold. In 1726 John Trethowan owned it.   After that the family appears to have sold its land to the Vyvyans of Trelowarren.

C.S. Gilbert (Survey of Cornwall ) in 1820 noticed a  " a shield over the entrance to the house which is supposed to bear the Arms of Trethowan , apparently three garbs.
" Trethowan farmhouse is in ruins and shows no trace of ancient work except some granite quoins. The shield has disappeared.

Trethowan is now farmed with Merthen.  There is said to be a crock of gold buried at Trethowan, and more than one vain attempt has been made to discover it."  The ruins are still visible and I do have a photograph of a print purporting to be the old manor.