This is not exactly addressing the OP's original question but it's proximal so I thought I'd mention it.
The larger regional RORO's such as the PEI ferry - I would expect, as the other posters have said, that those guys know what they are doing and can safely load just about anything. There's another recent thread about ferry options to NFLD and none of the folks who made that trip mentioned any physical restrictions on loading where trailers are concerned.
Smaller ferries in the same general geographic areas - not so much. For instance my husband and I took a look at the Englishtown Ferry in NS, which connects to the Cabot Trail and is one of seven smaller ferries operated by the province. We flew this year but plan to drive next year. I don't see us getting our 22-foot Airstream Interstate safely onto that ferry. Even in the best tidal conditions, the ramp angle changes too abruptly, meaning the rear end would scrape or hang up. We will be doing the circuitous route instead.