The batteries are in trays underneath the van. There is no more space to add additional unless you have some kind of bracket made, or run cables to a location inside the van where you could mount another battery or two - not ideal.
One thing you could do is change over to lithium drop-in style batteries. Stark Power makes a great battery. What you would get is essentially double the battery capacity. The downsides to doing this are that you would need to change out your converter to a lithium compliant one (~$200), and you can't charge them below 32 degrees F. So if you like winter traveling, then this is not for you.
WGO uses the Zamp solar equipment. There should be a combiner box on the roof with 3 inputs. So if you are getting the 200 watts, there is one open connection on the box. If there is room on the roof to mount another panel, you just plug it in - there is nothing left to do - no settings on the controller to worry about. So it's just a matter of figuring out if you have the space, mounting it to the roof and plugging it in. I would recommend you purchase a Zamp brand panel and make sure the voltage is the same as your other panels - they work most efficiently when matched.