I don't believe you can sync to SFDC campaign fields from Marketo. The only way I can kind of think of to accomplish this would be to create a custom field at the lead/contact level (let's call it RSVP_Number). In the registration flow for an event, you'd add them to the SFDC campaign and you would have a wait step (say, 10 minutes), then clear the RSVP_Number they just completed.
In that 10 minute span, you'd have a workflow rule in SFDC fire for when a lead is added to a new campaign and it would copy the RSVP_Number value from their lead/contact object into the RSVP_Number campaign custom field.
I'm not positive this would work and I've never done anything like that before, but just off the top of my head I think it might work, or at least give you some ideas of solutions.