The screws in the ribs provide a thermal bridge from the outside coolness to the inside warmer air. The water molecules in the warmer air stick to the cooler metal... condensation.
The fir strips need to be secured to a wood or plastic stand-off that is in turn secured to the ribs. This is easiest to achieve this is with a short stand-off mounted to the sides of the metal ribs of the bus. Then the fir strips attached to those. This means no screws contact outside metal will contact inside warm air. The screws holding the stand-offs should be sealed under spray foam.
In fact, any metal which touches or is bridged to the outside should be seal with something, Spray foam is the best for areas needing insulation. I like bed liner for areas like around the driver and dash...etc.
Of course, something like a mini-split will pull the moisture out of the air and will greatly reduce, if not eliminate, condensation.