Where are you from? You might raise the ghost of Wallace himself , and if from south of the border , might not end well.
Haven't been to either car park recently , but would think the Wallace Memorial would be the less likely to be moved on if there are no signs about overnighting.
Whatever you decide , ENJOY , but if you hear freedom being shouted , get the hell out of there.
If you travel further along the road to Blairlogie there is a parking area on the left, which Might be ok to stay on. The car park at Wallace monument often has motorhomes parked there when I drive past in the morning. I assume they have overnighted as the blinds are often closed.