When the tide is going to flood the carpark, the signs will be up to tell you what time you need to vacate and move to the campsite further down the causeway. This is a grassy field, and you can still see Mont St Michael, especially nice when lit at night. If tide is not coming in, then you camp in the car park itself. I can't remember how much it was last year, but not terribly expensive.
There is a hotel you pass just before the grassy overflow campsite and last year this is where you could get free wifi.
It costs 8 euros to park all day and one night,most of the area for motorhome is mainly gravel.Plenty of signs about to let you know when you can't park due to high tides,but if this is the case plenty campsites nearbye.
At the begginning of the causeway as you enter the town area on the right hand side is a large parking area. Perhaps you could stop there for the night. I know I did a couple of years ago. It is about a kilometer walk to the Mount.
As Geo has pointed out the Mont is a European Heritage site and dredging work is to be carried out so as to allow the tide to completely encircle the Mont.
This will mean the end of the parking facility alongside the Causeway.
There are details of a nearbye site for motorhome parking on the following link, look in the lodging page and click on camping.