Since it is a NEW motorhome, in order to license it in ANY state, you will need the Manufacturer's Certificate Of Origin. You will also need any lien paperwork and/or loan papers. The lien must be entered on the title, as well as the lien holders name and address. In some states the title will be sent to the lien holder, in some states it will be sent to the registered owner.
Yes, as previously stated, you WILL need a Florida address to register it in Florida. It will probably have to be a street address, not a Post Office Box.
A PMB at a UPS store may do the job. Your address would then be your name, PMB XX, the street address of the UPS store, town and state, such as:
John X Bacigaplonski
PMB 246
789 Any Street
Any Town, FL ZZZIP+FOUR
We used just such an address for about three years when full timing. No problems at all in Montana.