There is now availability showing on the Tohapi website for pitches at Riva Bella but for a minimum 2 night stay. However it is still possible to book a single night through the Caravan Club website, though you then need to phone CMC to inform the site of a late arrival or an early departure from/to the ferry, or if you have pets. Still no answer from Tohapi to my email about whether they now accept the ACSI card at Riva Bella, so we decided to book a single night stay for June via the CMC - at £19.26 including 1 pet and the tourist tax it is cheaper than the per night price booking direct and not much more than the 18 euros per night ACSI price.
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I’m sure it will be fine. Don’t forget to phone CAMC once you’ve booked online so they can inform the site that you’ll be arriving off the ferry. A bit of a pain, but booking online saves the £10 charge for booking through the Contact Centre and we didn’t actually have to wait too long before we got through on the phone.