Truan family:

[maryanne]

All though this isn't anything to do with the Truan/all spellings it has the name Peter Harris and Ann a sister.
http://www.cornwall-opc.org/Records/parishes/A-D/cuby_will_harris.php
Tregony is in the same area so maybe Peter Harris Trewen is a son of a Peter Harris from that family,hence the name.
Clutching at straws most likely.
Added:
St Just in Roseland -15 Feb 1801 has a baptism of Thomas Trewan -mother Ann but no father. It is noted-natural child.