Towing tyre pressures

I know this issue has been done to death before, but my topic search seems to throw up 305 threads, all but 5 being archived - and I can't find the active ones (I must be doing something wrong)! Specifically, I am seeking feedback on why my rear car tyres seem to increase in pressure by at least twice the amount that the front tyres do, and the tyres on the caravan do. The pertinent facts areas follows:
Car: 200 Series Landcruiser turbo diesel weighing approx 3460 loaded (without caravan attached).
Suspension: has been upgraded by ARB with 200kg system (heavier coils front and rear and OME Sports shockies at each corner).
Rear coils have Coilrite, inflated from 12-16psi (still experimenting).
Tyres: Bridgestone D694LT 285/65R17 (on both car and caravan - approx 9,000km since new); load rating 1400kg each.
Caravan: Free Spirit off road; weight approx 3100kg loaded; Simplicity 4500kg load sharing suspension.
Hayman Reese WDH 750lb bars, with 100mm drop hitch.
Ball weight of caravan on car = 280kg.
Tyre pressures: Car - 40psi front; 45psi rear. Caravan: 40psi all around
(Note: this is for bitumen highway driving. Bridgestone fitters recommended 40psi all around).
All wheels and tyres have behaved as expected (approx 4psi increase when hot). Rear car tyres increase by 7-8psi from cold when towing - this is the problem.
This happens despite the WDH transferring some of the caravan load (ball weight) to the front wheels. It also happens on gravel/dirt roads when I remove the WDH bars for slow going over very uneven ground and creek crossings (e.g. on the Gibb River Road and Savannah Way from Roper Bar to Borroloola.
I am unsure of the recommended maximum inflation pressure for these tyres and am a bit wary of e.g. increasing cold pressures at rear to 50psi to see if this makes a difference. Dropping them to 40psi still results in them increasing by 6-8psi when hot. Any simple suggestions to get a more acceptable rear tyre pressure increase when hot? Mike