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.