I added three more panels to the RV roof. They had to be mounted in a way that the mounting brackets could be screwed into the joists, which are spaced every 18 inches or so:
We now have 555 more watts of solar on the roof. Adding brackets that allow the panels to be tilted towards the sun, increasing power production by 30% or so. I decided it was easier just to add an extra panel.
Next I have to add the solar change controller and wiring.