I'm not familiar with U-hauls, but as Lizbeth pointed out, it depends if your
windows are leaking around the outside frame, or within the panes themselves. Layers of silicone sealant around the outside frame, for example, are a clue that there could have been long-term leakage that was just masked by a previous owner. The right way to fix this is most likely to remove all silicone, remove the window, then reinstall it using butyl tape, and you should be good to go for another 20 years.