I've been working with glass for 40 plus years:
No need to replace the whole window. You can replace or repair just the glass.
In RVs, the hardest thing can sometimes be getting the durned thing out of there.
Being the part that opens is a bonus. Easier to remove.
I'll assume you can figure out how to remove it.
I've repaired a ton of fogged units over the years. Repair vs replace depends on how long it's been fogged up. If it's been a while, a few years, the fogging inside will typically etch the glass permanently. Then the whole unit has to be replaced.
Both sides of the sealed unit should be tempered glass.
If it hasn't been too long, there is a chance you can repair it.
The glass shop (or yourself) can cut the unit apart with a sharp utility knife. The spacer bar should be replaced always. There is a desiccant inside the spacer bar. Cut the unit apart, wash the fogging off the glass thoroughly, if it will in fact wash off. Razor blade the old seal off too. New spacer bar, reassemble and properly seal it. It takes a special compound to seal it with, and specialized equipment to apply it. Polysulphide or silicone are 2 common sealants used. In a glass window application, silicone glazing is better than polysulphide. It is more resistant to UV and vibration issues.
Worse case scenario, you take the unit in to them and they have to order a new one. It takes a place with a glass line to produce a new one. Most typical glass shops don't have that. Window manufacturing plants do.
I have actually done these repairs on a piece of cardboard on the tailgate of my truck. Not the best, but when you're stuck, you improvise!
Don't let anyone tell you it's a vacuum between the panes. It isn't.