Ex-Australia assistant Muggleton departed Parc y Scarlets at the end of last season but Priestland remains grateful for everything he learnt.
"I did a lot of work with John Muggleton down at the Scarlets," said 23-year-old Priestland.
"And he was a big fan of a 10 that could run. So I've tried to develop it last year and more so this year.
"But I've always enjoyed running with the ball, it's just that maybe getting a bit physically stronger has helped me."
After being ear-marked as a future star since a youngster, Priestland has enhanced his reputation this season and helped the Scarlets to second in the Magners League.
With British & Irish Lion Stephen Jones occupying the number 10 jersey, Priestland has been forced to play at 15 but has slotted in at fly-half in Jones' absence.
"At the start of the season I wasn't too keen on playing full-back," added Priestland.
"But I ended up being on the bench for a month so I realised then, pretty quickly, that it's better to be on the field in the action than not playing.
"So I've enjoyed the games I've had at 15, but at the end of the day I'd rather play 10, but if I get asked to play 15 I'm not going to turn it down."
Guinness PRO12
Suite 208, Alexandra House,
The Sweepstakes
Ballsbridge, Dublin 4, Ireland