Forest River has seemingly partnered with ZAmp Solar. Almost all their new RV's are coming out as "Solar Ready". In reality, IMHO, it's a marketing ploy to get you to buy some ZAmp solar panels.
The external plug should be hard wired to your battery. If you want to add solar to your RV, you just need the panels and a controller to plug into the ZAmp circuit.
But it doesn't really make sense if you want a roof-mounted system. The plug is on the outside. So, if you were mounting a permanent system, you would want the wiring to be hidden, not laid down the side of your RV. But, if you wanted a temporary system, the external plus would be just fine.