I think you'll find all the answers above are not correct, quote from Gary below in a post i added to the search button in my post applies to all types of trailers, tyres and vehicles.
Quote: Originally posted by arc systems on 11/8/2006
The way to work it out assuming all tyres are the same is
one tyres max pressure divided by total maximum weight the two or four tyres can carry, multiplied by total weight of van
IE, 175R13C 6PR with a load rating of 94 equating to 670kg per tyre at max 54psi and a single axle van at 1136kg max
2x670=1340, 54/1340=0.040 0.040x1136=45.44 or 45psi per tyre
It is also recommended that the maximum load stated is reduced by 10% when using a tyre on a caravan, this is to take account of the use it gets and standing in one place for large amounts of time. The tyres above are fully rated at 1340kg per pair, so this then should be reduced to 1206kg