Buy ferry tickets now or as soon as they're on-sale.
Buy the camping and entry tickets direct from the ACO:
https://www.lemans.org/en
And there you have it. We stay in Epinettes campsite every year for the main 24-hour race, there are always motorhomes dotted around. It's a nice campsite too, with some tree shade and close to the tram stop to get into the city centre. Last two times I went to Classic Le Mans I stayed in the Maison Blanche campsite, which has no shade and while right next to the track, is nowhere as chilled out as Epinettes.