The hosts started brightly and found themselves 17-0 up before the 20 minute mark.
Jake Goodridge scored the opener before Priday crossed for the first time after a burst from 30 metres out.
Kieran Sparrow then went in under the posts for the third of the day before Trowbridge finally stopped the bleeding with a converted try of their own. 17-7.
Then came one of the stand-out scored of the afternoon as a set-piece move enabled Priday to slice through the defence and score in the corner for the bonus-point.
Trowbridge did reply with another converted try however, and the scores were 22-14 at the break.
It was another rapid start from the home side as Jack Pinker stepped his way over the whitewash and his conversion opened up a 29-14 lead.
Despite looking the better side for the majority of the match but two unanswered tries from the visitors brought the scores to within a point.
But Thornbury would not be denied and they finished strongly to take the game away from Trowbridge.
Fin Macleod-Bailey dotted down in the corner and Priday got his hat-trick when he scored from a driving maul.
Alfie Richardson put himself on the score sheet late on, which proved to be the last of the day and the conversion saw the scores read 46-28 to Thornbury.
S second win in a row for the Thorns sees them move up into third in the Regional 2 Tribute Severn table, with two games in hand on the top two, who are one and 22 points clear.
Elsewhere, Thornbury 2s beat Dings Crusaders 3s 61-33 at Shaftesbury Park.
Jack Oakey scored four tries, Stuart McWhinnie three, Dan Green scored a couple and Charlie joint also touched down.
Joel Bezus converted seven and Jack Oakey one.
