You need to do some math before you make any decisions. What is the expected increase in output to the batteries with a battery to battery charger? Will it be enough? Are you driving enough to justify the expense and to actually get a good charge on your batteries?
The good thing about solar is it charges whether the engine's running or not (assuming there's sun on the panels, of course). I would probably either switch the frig to LP or just add more solar panels to the roof, or both. How many watts do you have on the roof now?
My house batteries are charged only by solar and I never run short of power. No connection to the chassis charging system, and I don't own a jenny. But then again, I use LP for the frig.