At first I thought this was a simple answer and the more I thought about it, the more complex it seemed. Let's start with the cause of the problem.
- Condensation on the ceiling - moisture in the air hits a cold surface.
- There are two solutions - raise the temperature of the ceiling about the dewpoint or reduce the humidity of the inside air - or a little bit of both.
Raising the temperature of the ceiling
The best way to raise the temperature of the roof would be to install some sort of foam on the exterior of the roof, thus bringing the roof inside the thermal envelope of the RV.
This is not very practical, so we need to insulate on the inside. No matter what we use on the inside, the temperature of the ceiling will remain cold (in St. Louis in the winter anyway) so we need to be careful as to what we use. Insulating with fiberglass batts, for example, will still allow air flow through the insulation and once it hits the roof will still have the same issue.
So we want to use an insulation that will also prevent air flow or act as an air barrier.
Not sure exactly how an RV is constructed, so not sure how accessible the area is - if there is ductwork in the ceiling area, etc. - so will try to give a few options:
- Spray foam should do the trick - there are many opinions on this but I would think a closed cell foam would work the best. An open cell may still allow moisture to pass and you could still have the condensation problem. The closed cell should not allow any moisture to pass. Not sure what this would do to the indoor air quality and/or the overall tightness of the RV - would want to be sure you have some sort of air exchange to avoid CO buildup, etc. This is also a semi-permanent solution so if it does not work for some reason - could have an expensive problem on your hands.
- High density fiberglass may work well if there is not great access - can be blown in though a few small holes cut in the subceiling. Most high density products are air barriers (over 3.5 under per cubic foot I believe will constitute an air barrier) so should perform similar to the foam.
You may have the same issue in the walls and both of the above options should work there as well.
Reducing the humidity
As for reducing the humidity - not sure what type of HVAC system is in there, if there is fresh air being introduced, etc.
Your options typically are a high efficiency HVAC that can dehumify the air but if your problem is in the winter, this may really help.
If it is tight construction - remember to provide good ventilation, bringing in clean, fresh outdoor air.