From about 1700 to the end of his days he prosecuted without cessation his inquiries into the topography and genealogy of Cornwall, and he soon made 'great proficiency in studying the Welsh and Cornish languages'; but he quickly became involved in pecuniary trouble.
To improve his property he obtained in 1706 the queen's sign-manual to a patent for a weekly market and two fairs at St. Agnes, but through the opposition of the inhabitants of Truro the grant was revoked.
Tonkin put forth in 1737 proposals for printing a history of Cornwall, in three volumes of imperial quarto at three guineas; and on 19 July 1736 he prefixed to a collection of modern Cornish pieces and a Cornish vocabulary, which he had drawn up for printing, a dedication to William Gwavas of Gwavas, his chief assistant (this dedication was sent by Prince Louis Lucien Bonaparte on 30 November 1861 to the Cambrian Journal, and there reprinted to show the indebtedness to Tonkin's labours of William Pryce).
On 25 February 1761 Dr. Borlase obtained from Tonkin's representative the loan of his manuscripts, consisting 'of nine volumes, five folios, and four quartos, partly written upon,' a list of which is printed in the 'Journal of the Royal Institution of Cornwall,'.
Tonkin's manuscript history passed from Lord de Dunstanville to Sir Thomas Phillipps, and was sold by Messrs. Sotheby & Co. for £51 to Mr. Quaritch on 7 June 1898.
Polwhele called Tonkin 'one of the most enlightened antiquaries of his day.’ A fuller biography can be found in an article by H. L. Douch titled "an appreciation of a neglected historian" published in JRIC ns iv 145-80