If the frame is fine, you should be able to get a piece of tempered glass of the right size and shape from a local glass place and have that put in. It's glued into the frame, somewhat similar to how car windshields are installed. A decent RV repair shop may have a better time of doing the installation than a general glass place (partly because they're likely to have the scaffolding etc. needed already at hand and set up, and partly because they'd have more experience replacing glass in RV windows).
Replacing the whole window unit is not too hard for a do-it-yourselfer with a helper or two, although the cabover front is an inconvenient place to work on. I think that replacing the glass in the window frame properly is a little bit more tricky for the inexperienced, partly because contamination with e.g. oils from one's fingers can lead to leaks.
The whole window unit itself is generally a two-part deal. The main part is the outer section, which is sealed to the outer skin with butyl caulking tape under the outer flange. It's held in place by the clamping ring on the inside, and the two pieces are attached together by a profusion of screws.
