Hi - you can buy electric ones if you park outside house but they will need emptying quite regularly or why not just buy one of those cheap ones from Wilko or B and Q that have water absorbing tablets inside? Sorry no details but all DIY shops have them
However, if you want to keep dry all the air in your motorhome’s interior, an inexpensive container of desiccant crystals will not be adequate. It might be OK for a small space - a wardrobe for instance - but for an Inca’s whole interior you would need a dehumidifier with much greater air-drying performance and you would probably need to run it continuously.
A motorhome’s interior is not well sealed, with the outside air able to enter through deliberatelly provided roof-window ventilation apertures and gas drop-vents, never mind the commonplace draughts caused by poor fridge installation, poorly fitting door seals etc. So, if the outside air is moisture-heavy and the air inside the motorhome is no longer being dried, the air in the motorhome will soon become as wet as the air outside..
You can try using a dehumidifier to combat mould formation on the ceiling but, realistically, you will need a good quality appliance and operate it not just for short periods of time. Meaco market a range of 230V-powered dehumidifiers
but I cannot advise on how effective they might be as I have never used any type of dehumidifier during the November-February 4-month period that my motorhomes have been static outside in all weathers in the UK.
Dehumidification is a topic that has cropped up many times within the touring caravan community. Unfortunately motorhomes and touring caravans are not sealed boxes and are designed to have some fixed ventilation. You must never obstruct this ventilation as it is essential for when we use them.
The causes of humidity, damp and condensation are often not fully understood. Briefly, air can carry an amount of moisture, how much depends on the temperature of the air, when it's warm it can carry more than when it's cold, consequently, when moist warm air is cooled, it has to get rid of some of its moisture so it condeses out as water droplets. That's what causes rain, and condensation on cold surfaces.
Conversely as air is warmed up it can absorb more moisture, which is why we use hot air for drying things. Similaremperature effects also apply to many materials used in caravans.
There are a number of activities that create extra moisture inside, cooking and making hot drinks, but do not underestimate humans and any animals. One of the most significant sources of warm moist air is what we all breathe out.
When we use caravans, we tend to raise the temperature of the internal air and of course the fittings. This makes all these things capable of absorbing some of the moisture we create so during use the vehicles moisture content will rise.
When we put it into storage, the vehicle will tend to cool down, and some of the absorbed moisture will be released , but as the air is also cooler it can't absorb it so it appears as condensation.
The vehicle will only release its moisture until the relative levels of humidity between construction materials and the air are matched. Once equalibrium has been reached, no further transference will occur.
The upshot of this is that there may be some benefit to using a dehumidifier for a few days after the vehicle has been used, but longer term it will offer no practical benefit over maintaining good ventilation. It helps to open all spaces to allow full ventilation.
If you have a persistent damp problem, it would be wise to check for any structural leaks and to resolve those rather than treating the symptoms.