A job like that sounds like something that will need a lot of beer after ;)
What will you make the new frame out of? A cut down version of the original? You'll want to make sure it is as close to airtight as you can get. That's almost the worst part of the original windows is how much air they leak. Double panes do double the r value from a whopping 1 to 2, but in day to day living I most noticed the drafty air leakage the stock windows have. ymmv
Are you sure you're cool with non-opening windows all around? That would bum me out.
It would be easier imo to just double up the whole window assembly. Then you could still open it and it would take a lot less fabrication. Not sure what your skill level is so that also is a factor. You could be a metal fab god for all I know, in which case, rock out.
You can pickup new double pane rv windows for a reasonable price on ebay if you decide to do something else.
At least you got nothing to loose!
Good luck, post some pics!
Patina enthusiast and professional busman