Vince,
There are several factors which affect condensation , the two most important being temperature and relative humidity.
You could increase the temperature with an in van heater.Warmer air can hold more moisture , i.e heavier rain in summer!
The RH , amount of water in air , can be reduced by using a dehumidifier or more cheaply a dessicant , such as silca gel.
These are dried in an oven and then placed in van to absorb moisture and process is then repeated as required.
Also removing sources of humidity , water in tanks , wet cloths etc will help considerably.
Another factor affecting condensation is the conductivity of the material , which is why JB's insulation works as it is a much poorer conductor than metal.
Hope this helps without getting too technical,
Cheers,
Doug
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