Condensation? Or is it a seam leak?
Condensation is normal, especially on really cold nights. Usually best alleviated by leaving a flap open a little bit to allow the moisture to escape.
If its a seam leak just use some seam sealer found at most decent sporting good stores where camping equipment is sold.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE