The option cost for DP windows is at least $1000-$1500, in many cases even more. Full replacement cost is likely twice that. Not sure if the benefit is worth that much, only you can decide.
On my last unit with single pane windows, I added a 2nd pane, but only on the fixed portion of each window. If you have large fixed glass areas with small vent glass, it will provide some benefit. I used 3/16 inch plexiglas and a rubber 'H' gasket from JCWhitney. Cost about $200 including Gila RV window tint film on all glass.