The first time i crossed the channel with the California , like yourself i did not know what to book as .
I played it safe and booked as a motorhome as i was concerned that i would turn up at Dover and have "issues" ,especially as i also had 2 bikes on the bike rack.
However , whilst checking in , i asked the gentleman in the booth the hypothetical question 'what would he have done if i had booked as an oversize car?'.
His reply was that he 'would not have questioned it' , but could not say that one of his colleagues might have queried it.
Moving on from this situation , i have since crossed the channel 3 times with the Cali.
Each time i have booked as an Oversize car , again with bikes on the rack. I have not experienced any problems at all so far and will continue book like this until advised otherwise.
My thoughts are that ,If i were to be questioned , the worst scenario would be that they would charge me the extra as a motorhome at check in and therefore i would be no worse off.
Have a great trip.