Qualtrics can randomly assign participants to conditions or blocks within a given survey pretty well. But what happens if you have several surveys and you want to randomly assign participants across all surveys?
When I needed to do that, I decided to create another survey which I called "randomizer". In that survey's flow, I added a Randomizer element that contained several End of Survey elements. In each of the End of Survey elements, I customized the End of survey message to be a redirection to one of my actual surveys (see image below).
This way, I could direct all my participants to one url, the one for the "randomizer" survey, and from there they would be randomly re-directed to one of my surveys.