Thank you - I'm feeling hopeful!
I'm a new user so I hope I explain this situation in a way that makes sense. I'm using iFrames that embed the Marketo form on a non-Marketo landing page. The campaigns activate when someone fills out a form, referencing the web page the form is on.
Here is the exact verbiage I received from tech support:
The Visits Webpage method will only work inconsistently. You will need to populate a cookie on the home page with a value that indicates which product they are filling out the form for, and then have a hidden field in the form which populates with the value from that cookie. You could call this field "Most Recent Product Interest" or something similar to indicate which product it is for without overwriting any existing information.