I don't think flannel will absorb any more moisture than a 100% cotton fabric. Having had several RVs over the last 10 years or so I can tell you that unless you have really good insulation and double pane windows
, you are going to get condensation, especially if you camp in the fall
, winter or spring in the Pacific Northwest. Here are some of the ways that we reduce (but don't entirely eliminate) the problem:
1. Use a good exhaust fan, especially when cooking, and keep a window cracked open a bit. Anything that will help to circulate the air is good.
2. If you're camping with electrical
hook-ups, use a small de-humidifier.
3. During the off season if you can store your egg somewhere with access to shore power, you can keep a little electric heater on the lowest setting to help keep things dry.
4. If you don't have electricity where you store your trailer, at least use one of those little devices with the dehumidifying granules in it and prop all your cushions up on edge so that moisture doesn't get trapped on the undersides. Better yet, store them in your house.
5. When traveling in damp climates, we dry our towels in the tow vehicle when we can and avoid cooking things that need to be boiled or steamed because that just puts more moisture into the air.
All in all, I wouldn't worry too much about what fabric to use, just go with what you like!