There is a large parking lot available near the abandoned former fort of the Chartreuse. It is no longer in service and is a 20-minute walk away from the center.
In late August 2020, we stayed here with another camper van. During the night, around 2 o'clock, the campers were hit by stones or possibly even shot at! There were multiple dents and even a hole in the camper van! Next to the parking lot is a large abandoned area of a ruined barracks, where many questionable individuals gather at night. It's a shame, because otherwise it's a beautiful spot.
There is currently a barrier that needs to be bypassed. However, you can still park on the sloped gravel area without any issues. Alternatively, you can park on the street just beyond the entrance in a spacious parking area frequently used by locals. It is a level and peaceful location, offering the same amenities for accessing the park and the city.