Apologies if my reply before was somewhat arrogant.
Car camping around the Trossachs, and elsewhere, is a massive headache, as I am sure you know. It is also over popular, therefore so many hundreds of spots that were quiet and useful are now full of drunk muppets with no regard for environment or local community. Half of Glasgow, Dundee, Falkirk and Edinburgh car camp in the Trossachs this time of year onwards. Massive overcrowding.
Going on a proper walk from the car or train, or heading further north is the realistic option. As anagallis_arvensis suggests, go further north and more remote – or think laterally – such as a ferry ride or heading south.
There are also some fab ‘semi wild’ sites, that allow fires etc, and support a local business with a couple of much needed pounds.