Your first step should be to figure out what you have now. Odds are it is a WFCO, and already supposed to be a 4 stage. Problem is they tend to not properly enter the mode, especially not entering boost mode.
If you have a deck mount converter, your options are opened up a little. But most of fifth wheels are going to have a combination power center with the converter built in. That makes it pretty painless to use a Progressive Dynamics 4600, which will install right into your existing power center. And it is an excellent converter.
But it all starts with knowing what you have now, including what amperage output it is rated for.