There are different types of windows.
Depending on the manufacturer, you may or may not have to pull the whole window to get the glass out. Residential windows are made so both the slider and fixed unit can be removed without taking the frame out. RV windows go both ways. Many times you have to pull the window to get the slider out.
But in answer to your question, yes, you can do just one.
Single biggest thing is getting the durned glass out of there without disassembling half your MH.
Have a look to see if you can get the glass out without removing the frame.
On edit, I think I misread your question. I'll clean my glasses....
Yes you can change from double to single glazed, but you need to compensate for the narrower thickness. The DG frames are made for the thickness of the sealed unit vs the thickness of a single piece of glass. The fixed lite is easier to convert than the slider part.