The first question would be: "Are you looking for a pseudo-storm window that helps reduce cold/heat loss in winter and summer?" or are you looking to reduce the heat gain from the sun shining into the trailer through the "overly large single pane windows" ???
If it's the first, then you'll need to determine which is the most desired outcome (condensation reduction/heat loss or blocking the sun/reducing heat gain) few solutions, short of replacing the windows with dual pane windows, will be a good solution for both.
If reducing heat gain is your objective, you might consider making "solar screens" for your windows. I bought mine at Home Depot and used industrial Velcro around the window frames to attach the screens after setting up the trailer. I remove them for towing and reinstall them as a part of setup. They block about 80-90 percent of the heat gain inside the trailer and it's noticeably cooler when sitting in front of any of the windows.
This is what I used. It's available in different widths and lengths, so only buy what you need. I've had them for about 6 years and they're still functional. I have had to replace the self adhesive Velcro in places on the trailer window frames over the years.
https://www.homedepot.com/p/Phifer-6...2232/100538470