It depends... it always depends!
This article is pretty good - and has nice visuals.
https://solarpanelsvenue.com/mixing-solar-panels/
It isn't the watts that are critical, but the current/voltage output of the differing panels.
The wiring is easiest if you want to hook two different panels/arrays up in parallel. Let's say a single roof mounted panel is one array, and wired in parallel. Simple, as the negative from the solar panel goes to the negative on the controller, same with other terminals, positive to positive.
The portable would then plug into the exterior plug on the camper, and those wires would also be going neg to neg, pos to pos. Thus, both panels would be in parallel. As the above linked article says, do this if the voltage outputs of the panels are close ... or at least more similar than the current outputs, since the current outputs of each panel don't matter if they are in parallel. If the current outputs are vastly different, then serial is likely the better option.
To do a serial hookup, the initial wiring is more complicated, as the wires from the rooftop panel need to go "through" the external port on their way to the controller. And you will either need a switch somewhere close to the external port that connects the two sides together when the external is not plugged in... and needs to be flipped before plugging in the external panel or sparks may fly.
I'd use a small dongle to plug into the external when the portable isn't plugged in to be the "switch". That way, when you want/need the extra oomph from the portable, you would remove the dongle and plug in the external. Voila, serial connection made.
Make sense? Pictures/diagrams would be helpful... I think. I might whip something up shortly, but no longer have any nice software on my PC to do that with....
