Hi Paul Welcome. JohanGPS reported here mapping-handhelds/rhd-on-montana-t255.html that that he loaded the RHD to his Montana without any problem using PoiEditor. That mean that the same process as for the Nuvi range can be used successful.
Paul here is a link that may help you through the process general-discussions-rhd/nuvi-42lm-t333.html
You will need to download the files Road Hazard Data Wave, GPS Sounds Wave, Toll One Sounds Wave and Toll One Data Wave form the RHD section.