I can only tell you what works for me, not what theory says I should be doing...
The following is from the NRMA caravan info site:
"4 psi rule":
"Tyre pressures - do-it-yourself check
It is impossible to list the correct pressures for every caravan, due to variation in size, load, etc. This easy check will help you find the best pressure for your caravan tyres.
1: First inflate the tyres to the pressure recommended by the manufacturer of the trailer or the tyre you are using. Secondly, tow your trailer for a distance of 100 km, preferably on a highway.
2: Recheck the tyre pressures immediately after pulling over and compare them with the pressures you had at the start of your run. If the pressures are right, the hot readings should be 4 psi (28 kPa) higher than the cold readings.
3: If there is a greater than 4psi (28 kPa) difference between these pressures, the tyre temperature is too high and the pressure needs to be increased. If there is less than 4 psi (28 kPa) difference, the pressure needs to be lowered.
4: Large 4WD tyres will have a differential of 6 psi (42 kPa).
5: Be sure to use the same accurate gauge for both readings."
And this one from another forum...
"Another way to find out approximately what pressure you should run is from a very simple formula derived from the tyre specifications:
max tyre pressure and max load rating are shown on the side wall of the tyre. Plus you need to know the weight of the van you'll be towing...
max tyre pressure divided by max load rating times actual load per tyre
for example: 450Kpa / 950kg x 450kg = 213kpa (30psi)"
I have successfully used the 4 psi rule for both a van with 13" Holden wheels with L/T tyres, and a van with 14" Holden wheels with L/T tyres. In both cases I've started with cold pressures of 40 psi. I've towed both those vans for a combined distance of nearly 12,000 kms, and haven't had any problems with tyres. I certainly wouldn't preach to you that you should do this or do that with respect to tyre pressures. I can only say what's worked for me.
Caravan,I've settled on 35psi. I'm taking the grandkids away camping leaving Saturday for Wakiti Creek Caravan Park ,near Echuca for a week. Unfortunately we don't own a VV so our Roadstar Little Ripper and an old tent will do the job. This will be the kids first camping experience and it looks like being very hot. One thing you can't choose is the weather, but I'm sure we'll have a great time.
Quite interested in this thread also. I have a 1964 Wayfarer Roadliner that weighs in at just under 1,000kg with gas bottle and full watertanks and packed. 100 on the tow ball. I have no idea what the manufacturer GVM for the van is. I currently have 185/R14/LT 102/100s tyres on the van but am having trouble working out the correct pressure with out knowing the GVM or MTPLM of the van. I have been running them at 35psi without problems so far.