We bought a brand new lemon that had apparently been leaking it's entire life on the lot. We could smell mold and mildew coming from the bedroom slide the first night. When they took the slide out the entire inside structure - floors, ceiling, walls were covered in black mold. They offered to "remediate" it for us, saying it became our problem when we took delivery. We threatened lemon law and got all of our money back.
We have owned 6 units (excluding the lemon) from pop-ups, to class Cs, a Class A and a travel trailer; some preowned, some new. Four have leaked, two have not. We maintained all of them faithfully. As others have mentioned, there are a number of factors to take into consideration. If you store in the north in the winter, water can get into the tiniest of cracks, it freezes and expands creating leaks. In the south, the hot sun plays havoc with the seals on the roof. Other than regular inspections, seal testing and using the RV wisely there is little to do to be assured your rig won't leak.