Nearest to the town is Hendre Mynach, or you might consider Ynys Faig Farm on the opposite side of the Mawddach estuary and just across the track from both Fairbourne stations.
There a few other possibilities listed on Camp Site Search, of course.
A day of all things motorhome and campervan. The show is held both in the main indoor arena and across the grounds. Catering, hot drinks and food options are available. Easy access to the site and free parking included. Win one of five pairs of day tickets