There are a few things that can help, but we need some more details on your setup.
First off the help,
We went the shrink wrap plastic on the windows route. Our windows have a 1/2" wide flange strip all around the window that we can attach the 2 sided tape to to hold the plastic on.
Like this
While it takes time to do all the windows with that film, it is cheap compared to making or buying storm windows. The plastic film keeps the window clear all the time, helps keep the heat in and the cold out.
However, the 1/2" wide flange strip is connected with metal to the outside, we go get a little condensation on the 1/2" flange. In the morning I use a dry wash close to absorb what forms overnight. And it is good the rest of the day until the next morning.
The next part is doing moisture control inside the camper. You stated you have a dehumidifier but you did not state what the inside humidity was with it running? What is it?
We have winter camped enough to sort out how to control the moisture, the learning curve created this list.
1. Shrink wrap on all windows except next to the stove.
2. When cooking or washing dishes, crack the only window not covered in film open to let the excess moisture out from cooking and dish washing. Close when you are done.
3. When showering, open the roof vent (a good 3 to 4 inches, in the camper in the bathroom. Let all that high humidity go right out the vent. Close when done showering.
4. Crank open a roof vent, in ours, it is the opposite end of the camper main bedroom. That vent needs to be open a good 1" up to keep up with the amount of moisture from 2 adults sleeping over night in a camper. Yes, you will loose some heat out the vent, but the moisture is wanting to get out, so let it go.
5. Run a big enough dehumidifier to drop the moisture in the camper down to 45 - 50% or lower. Depending on the situation, we can sometimes close the ceiling vent if the humidity is 45% or lower.
6. Get a humidity meter with high/low readings saving and out it in the camper. See one of ours. This is from the first day we went camping. You can see the max of 80% to the lowest of 45% on the first day. It scans over 24 hours to give the max/min. The 80% was from before we went into the camper.
6. The last upgrade I did and has worked really well, install attic vents on the roof to vent the space between the ceiling and the roof top. This helps a lot.
You really have to control the moisture in the camper when the conditions are right to create that heavy sweat on the windows. Higher humidity naturally seeks areas of lower humidity. It will migrate by itself or try to, which is outside. If you do not lower the humidity in the camper, it will seep up into the camper attic and be trapped there as the roof membrane has no vents to let it out like houses do. Left let go like this long enough, mold will start in the attic. I have seen it on campers where I replaced the roofs on them. Adding the attic vents lets the moisture get out and helps lower the issues in the attic. They also help let the high heat get out in the summer trapped up there.
The attic vents look like this when installed. You need enough of them to align with the length and way your camper is made. Some I use 3 vents on, others 4 all depending on the camper size and floor plan.
I use the JR Products brand no. 02-29125 Mushroom Style Plumbing Vent. Amazon and Ebay sell them. Prices are all over the map this year. Pre-covid they were 30% cheaper.
This link will take you to my Flickr photo hosting site showing them being installed.
https://www.flickr.com/photos/camper-johnb/sets/72157717497136833/Hope this helps
John
2005 Ford F350 Super Duty, 4x4; 6.8L V10 with 4.10 RA, 21,000 GCWR, 11,000 GVWR, upgraded 2 1/2" Towbeast Receiver. Hitched with a 1,700# Reese HP WD, HP Dual Cam to a 2004 Sunline Solaris T310R travel trailer.