Port grimaud holiday marina. I think they have a couple accessible from the back entrance.
You will pay well for it though.
St tropez a boat taxi ride across the bay
Along that port grimaud coast is a few sites some may have big pitches if you can email them.
We had little luck stayed h.m for 3 nights
European tour went through there 2 years ago. Soulbuddies had to wear the old buddy smugglers around the pool area. We all fitted on the pitches ,which had their own shower and utility room. Access was indeed from the back door
We have stayed at the Holiday Marina a few times. Good access via rear gate. Last year a major investment in facilities and pitches. Shower and bathroom on all pitches. Only problem is small brown bugs everywhere!