Front facing windows are optional on all of our models but discouraged.  Especially the front center slider window.  We can almost guarantee that a slider window will let water into your camper if you are driving forward during a heavy rain.  The gutters where the glass slides, can fill with water faster than the weep holes can drain so when the gutters fill up, the forward motion of the truck pushes the water into the camper.  We tell everyone to put towels down if they are driving in rain. The front window should not leak otherwise. Your other option is a fixed glass window.  Same size, but no chance for a drip or a leak. Third option is no glass at all.  90% of the campers we are building now have no window in front, which we strongly recommend.  The inside of the camper looks much better and keeping the camper comfortable in extreme temperatures is so much easier when you do not have a window just below the cabover.