Our old RV
Seen Here was about as leak free up top as one could get having a seamless double-wall honey-comb body. It had just one 9" diameter hole in the roof for a little pop-up vent and it never leaked.
But being naive 25 year olds with a first motor home, we were ignorant when special ordering that unit. The optional front cab-over windows didn't need to be. We thought they would be fun to have but they were nothing but trouble for the first year until I replaced them with a second set that I handled myself differently. Oh how painful that was. We kept the plexi-glass blocked with a back-packing sleeping pad for insulation & privacy, and piled up blankets & pillows all the time for safety since the kids liked to hang out there while in transit......please don't be hard on me over that last comment. It was different time under different laws.