If you are booked on the Manannan, then your booking will be of no use, as they put a mezzanine floor in & have a height restriction lower than a T5.

In addition, if your T5 is a van, then the cost will change (upwards!) if a campervan/motorhome then, as it is less than 5.5 metres, it will be the same.

When I was in a similar situation, I e-mailed the Steam Packet & got a reply in a reasonable time frame - i suggest you try that.

Congrats on your purchase of a T5, I had some great times in mine