Water inside the walls usually causes the hard bumps that can sometimes look like a rash. Feel along the wall to see if you can feel any swelling. When the styrofoam insulation gets wet, it will swell and is sometimes hard to see, but you can feel it. You may have a leak on top. I would inspect the caulking on your roof to see if you can spot where it could possibly be leaking.
You also need to look on the outside to see if you see any "bubbles" or a "wavy" look on your sidewalls. That will be a sign of delamination which a water leak could cause.
If you can't find anything that you can take care of, it's best that you get it to a repair shop as soon as possible. If it is a leak, things will only get worse.
Good luck and I hope this helps.
Gary & Trish
"PT Crew Member Since 11/2010"
2017 Prime Time Crusader 315RST TE 6.0 : 2007 Flagstaff 206ST MAC Series
2012 F-250 Lariat : FX4 : CC : SB : 6.7L Diesel : 3/55 Gears
2016 Nights Camped - 9