If you can afford the dual pane as a replacement, then yes it will help for sure. You will give up any crank out windows though if you have them. I have not yet seen a double pane crank out. All sliders. Means no window open when raining.
We winter camp and for about $10 bucks you can do an entire camper (32 feet) with the shrink wrap. It is one time use, but again it is cheap. It will last until you want to take it off, even a year or more later.
You can see here how I have done it. I have a combination of single hung, picture windows and crank outs. For the crank put, just take the knob off for the winter.
Start with this
A typical shot of the combo windows
You attach it to the inside metal ring around the window, not the wall board.
And the end results
Learning the hard way...I figured out I need to put this on "before" cold weather sets in. When the aluminum ring is really cold the double sided tape does not want to stick so well. At 50 to 75F degrees outside it works really well. As you can see from the dates, doing in late Dec still does work, just it works better when warmer.
The haze you see in the pics is the fog on the glass before I put the wrap on. That will evaporate out the leaking cracks in the window and then it is crystal clear. You can see here when camping in the winter with light snow outside the window is clear.
I will say this, if you are going to do cool weather camping, start investigating about venting the camper and dealing with internal moisture. The shrink wrap will keep the windows clear and help insulate but you still have to take care of the moisture. We vent and use a dehumidifier. You can see here in the morning condensation condenses on the window flange. I take a towel and wipe it off every morning, but still have to deal with the over all moisture and keep it under control. Get a humidity meter to gage to keep track of how good or not you are doing.
If the shrink wrap is not your thing, there is an option of storm windows and these are as effective as dual pane. They are single pane glass that goes right on these windows with a foam seal to the window flange. This allows crank outs, single hung and picture windows and not having to change the window you have now. Down side is you have to take them off and on and store them. I know they make them, I even have pics of folks who have them, I just have never bought them. That may be the cost issue. You can make them yourself from plexi or lexan if you are a handy wood worker.
Hope this helps
John