I can think of a few but, to me none are really valid.
Your trailer may not have all of the heavy things centered over the axles like frig, kitchen cabinets, storage tanks, etc? They may have thought pulling with a wagon or car might put more weight on the front tires because the nose is down?
I agree with your tire guy. The number on the sidewall was not arrived at in a careless, flippent way. That sidewall inflation number will support the tire to match the contour the engineers want the tire to run at. Tread flat on the road, beads pushed firmly against the rim, sidewall flexing but, not deformed, etc. The manuf's do have on their web sites inflation at load charts so that you can work backwards and lower your pressure to obtain a softer ride. If your trailer is light relative to the max gvwr of the axles and tires, you can reduce inflation.
I run a load range D tire which is 65psi & 2540 lbs max loading. I actually inflate to 67 psi cold and lose about 1-2 lbs per month. I stayed in Newfoundland for the summer and since it was cool, didn't have to add air even though I had slipped to about 62psi.
I would make sure the trailer is level when towing, inflate to sidewall psi (unless your argosy is a shorter one and you have a lot of reserve load capacity) and ignore what the plate on the trailer says.
Just my $.02