As mentioned above, when you check in be sure to mention what you are driving and how it's quite low and you fear of not being able to make it up the normal ramps. The last thing they want is a stuck vehicle holding back boarding and/or departure so they will ensure you are sorted.
I have done many crossings and did the above with the ferry company then having me board either with camper vans, being given a spot on the main deck etc.
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