Our 2002 Sunline had a front window that opened (jalousie style) and it never gave us any problems. I would look for a crank out type window....not a slider as I think it would be less prone to any type of water leak. I wish our Minnie had a front window as they really help with ventilation. Instead, I'm going to be installing a Maxx Air vent system.
A quick Google search for RV windows will bring up several companies that sell windows and give pretty good directions on how to measure windows.
Easy RV windowsHehr windows and others