Hello,
I have worked before in this scenario, with pages that load content dynamically it is mandatory to set up custom JavaScript to interact with the google analytics API, this could be done to track certain events as done usually with tags, or simulating page views with the action the user is doing.
So, overall the way it would work is not by changing the URL, but by sending pageviews to the GA api on each step the client has taken.
I would need to edit the OnePageCheckout controller and templates in your prestashop installation. If everything goes well, this could take at most a day, but depending on the structure of your OPC module, this could extend to 3 days.
I would be glad to work with you, regards.