Condensation on upper inside walls ral cold morn

I just looked back at your profile. We lived south of Syracuse for 50 years. You might want to reduce the fan temp that I mentioned to 75 or 70, or you might not move enough air when you're not around. IMO, real damage occurs when people leave their RV closed up tight when not in use after an outing and they leave all the produced moisture trapped inside.

luberhill wrote:
So can I just open a ceiling vent in the bathroom to allow movement ?
It just seems opening a window when it’s 49 degs and raining out is adding to it
Having to run the ac fan all the time would have me selling this thing tomorrow.
Running a vent fan all the time would be heating the outside and cause the furnace to run all the time...
Has to be a better way

Well at least you're providing humour to the day with your many posts. :W As a newbie you've been recently posting quite a bit about RVing issues that may be totally new to you but are old news to the rest of us who have been at this for awhile. Sidecar said it best - "How it works, like it or not." Believe it - you're no different than the rest of us and the solutions to these various issues won't be any different for you than they are for us. In this particular situation actively venting the trailer is the answer. ;)

luberhill wrote:
Has to be a better way

You need to examine the sources of moisture, other than breathing. Cooking, long showers..etc

luberhill,

The cause of the condensation is the walls are below the dew point temperature. If the walls can be made warm enough the problem disappears. Running a fan to circulate air may help to raise the temperature on the surface of the wall.

One other solution is to reduce the relative humidity inside the RV.

The easiest way to do that is to open a roof vent a bit, find the furthest window from the vent and open that a bit. An electric heater near the window will improve that and keep you warmer. I'd suggest a low wattage radiant type, or a small oil filled (if the oil filled can be on a shelf so the air from the window passes over the fins.)

I know opening a vent and window when it is cold seems counter intuitive, but it does work.

Another possible solution is an electric dehumidifier--but the cooler it is the poorer they work. They are generally energy hogs, some what bulky and definitely noisy.

Get a small dehumidifier, not dry-z-air, and electric unit. We have zero condensation in cold damp locations, spent six months at the Oregon Coast.

You have to kind of play the averages and stay aware of the situation. You will see mildew long before things start to "rot". Things can be damp in the morning, but we make sure we dry it out during the course of the day. Check cupboards and back corners of closets. Clothes can actually get wet if you let it go too long. Just watch the corners and edges and keep the air moving as best you can when you can. We kept "damp rid" type stuff in the closets and bathroom when we were full time. Also, below freezing you have to unhook your water and drain the hose over night. When not using the RV, I keep the roof vents and the little window above the sink open and the fans set to come on around 80 degrees.

Running the ac fan will just circulates air right?

You could get the same results with another kind of fan.

Bottom line, you have to move that moisture out side some way.

So can I just open a ceiling vent in the bathroom to allow movement ?
It just seems opening a window when it’s 49 degs and raining out is adding to it
Having to run the ac fan all the time would have me selling this thing tomorrow.
Running a vent fan all the time would be heating the outside and cause the furnace to run all the time...
Has to be a better way

A recirculating ceiling fan keeps air moving, which helps. But if your RV is small enough, just breathing will condensate.

luberhill wrote:
Opening a window at 40’degs outside ?
Ceiling fan on all the time also would take all the heat out..
Guess that doesn’t make sense to me..
Would I be better off going back to letting the ac fan come on when the furnace comes on

How it works, like it or not. You have to evacuate the moisture you, your occupants and everything else is making inside, to the outside or your RV will rot from the inside out.. It's not a house with good insulation so there is a temp difference between the outer and inner walls and you get condensation.

Vent it agressively or pay the price of rot down the road.

Your choice. Pay now or pay later.