The Roadsters and Touring models are actually CANBUS, Tony - only the std/classics are not.
You have a real task ahead of you getting over the CANBUS interlock - the problem is not getting the analog signals but overcoming fact that OEM gauge is not connected and bike will not start without it.
Only thing I know of is the Motogadget m-tri - not available for Rocket specifically but you MIGHT be able to get one of the other models to work
m-TRI Signal Adapter für Triumph
It is quite expensive for what it is!!!!
Other option is to leave your OEM gauge connected under your tank somewhere .......