Hi GMJ.
Went to Frejus around 2016 but stayed on a campsite just outside, ( Camping Caravaning Le Frejus ) on our way to Cannes, handy as there was a bus stop just outside into the town, I recall we missed the bus back but taxi was very reasonable.
Must say did not take much notice of parking as use public transport sorry i'm not much help, the amphitheatre is not to far from the site tho.