When my side window got broken at the end of last year, I asked myself the same thing. Sure it can be done, although depending on the the window type, with efforts that may include full removal of the frame from the camper. I found for the cost and time, it was best to replace the entire window in one shot. My tinted horizontal sliding window complete with retaining trim ring was about $60 from Forest River.
*Current: 2005 Ford F350 Crew Cab Dually 6.0 diesel 4x4*
*Retired: 1987 F350 Crew Cab Dually 6.9 turbo diesel
2007 Forest River WildCat 32QBBS