Very typical. Any tent or similar sewn outdoor assembly needs seams sealed. Personally I like the Seam Grip for backpacking tents and Sil-Net (almost a lightweight silicone caulk) for the fairly heavy urethane-coated nylon in my WilderNest but you may want to consider using plain old vinyl cement depending on the material in your camper.
not sure what year camper you have but I find in my 04 grandby that water will find its way in through the velcro when the top is down. water works its way into the folded canvas then runs back till it hits the velcro windows and wicks its way in. not large amounts by any standard but enough to get a mildew smell sometimes.
These campers take on water. It has been that way since the beginning, and the design has been improved marginally, at best, over the years.
Make sure the velcro is sealed up as tight as possible before you drop the top, and make sure your seals are clean and intact all the way around. Sure, you can add some seam-sealer. Other than that, it is what it is. These are "old school" campers that are still being built.