The biggest thing to look for in replacement motorhome windows is that they are the right size and shape for the make and model of RV. If the manufacturer no longer makes replacement windows for your model, you may need to have a custom replacement window made.
Consider what kind of glass you want, too. While it may be somewhat more expensive, laminated glass resists fogging in almost any conditions, so it can be a good choice for high visibility. Insulated glass units may be less expensive and provide good weatherproofing.
Look for a replacement window that comes with all the parts you need, if possible. Check the rubber gasket surrounding your existing window before you order a replacement. In most cases, you do not want to replace this. However, in older RVs sometimes the rubber is cracked or dried out, in which case you should find a window with a replacement gasket. You may need to provide some additional supplies, such as a putty knife and butyl putty tape.
Check your insurance policy before ordering your new windows as well. While most people can replace their own RV windows, some policies do cover professional replacement. If you do choose to take it to a glass repair shop, look for one that specializes in RV windows.