When I see a sign that states “No overnight parking of motorhomes (or whatever term is used) between the hours of X and Y” the hours are normally 11pmish to 7amish give or take an hour. No overnight parking means what it says. Normally spotted on sea fronts or in car parks in popular tourist destinations. Best to find a quiet spot in a side street and park up there overnight and then move back to grab an attractive day spot early morning as soon as regulations permit.
“No overnight sleeping” a bit more problematic. Enforcement depends on time of year and how “urban” the car park is. Midnight to 6am could be an issue in these circumstances. If it’s a pay car park then a blind eye may be turned off peak as it means overnight revenue for whoever owns the car park. This one is a judgement call and it really depends on how comfortable you feel when you park up overnight.
I guess what is not wanted by the land owner is a parking area that fills up with large groups of “travellers” and regulation permits whoever owns the land to carry out enforcement if necessary.