Ann,
It seems that you have knocked on every door I can think of.
As usual, from one census to the next, ages are all over the place.
However there is a Thomas Cubbon burial, on Brian Lawson's site :
Jan 31 1880 aged 76. This would put the birth in 1803/4 and could correspond to Thomas ch. May 24 1803 son of John Cubbon and Joney Gawn. Joney (Judith) was buried on August 31 1832. Her will is probably on LDS film 106433, and may be worth a shot.
There are two John Cubbon wills 1803 LDS 106408 and more likely 1805: LSD 106410.
I hope that this helps.
Good luck
Bruce