The bumps are probably hard, they are black mold that grows under the vinyl wallpaper when wet.
Look for a leaking screw under the trim where the rubber roof meets the siding. Usually those screws can work loose over time, and the water will travel under the trim until it finds that screw. You should be able to take out the vinyl trim strip and find screws underneath it.
Steve and Vicki
2011 Dodge 3500 CTD Outdoorsman
2013 Columbus 320RS-ITS HERE!!