I live in Tasmania and travel on the ferry several times a year.
You do not need to be registered as a motorhome but you need to make your booking as a motorhome. If you don’t you will be treated as freight, the stevedores will drive your vehicle on and off and you will have to wait in port to have it unloaded, may take a couple of hours. The cost for an Isuzu NPS, 6.3m length was $1200 in Nov ‘18, ONE WAY, just for the truck, passenger fares were extra.
As a contrast I have rebooked a return trip as a motorhome with the same vehicle (camper is on) in May ‘19 and the cost is $800 return with 2 passengers and accommodation. The main reasons for the difference is vehicles travel cheaper than freight and the federal govt has a subsidy on motorised passenger vehicles but not trucks. The Isuzu qualifies as a passenger vehicle when configured as a motorhome.
If you are towing a caravan or trailer there is no subsidy for the trailer part of the vehicle as it is not motorised, the length of the trailer is calculated and you are charged full price for the trailer.
Pete