Let's take this a bit deeper...
There are multiple wheel sizes, tire sizes and tire manufacturers used on 25' FB Airstreams from the factory. Even more are in use based on what we owners do after we buy them.
My experience is with the 16" wheels and Michelin LTX M/S2 in size 225/75R16. Other configurations (e.g.: Goodyear Marathons on 15" wheels) will have their own answers.
Tire inflation is best viewed as a response to load... not a single answer for all situations even for a specific vehicle. Fortunately for us Michelin users, our tire manufacturer publishes detailed load/inflation tables.
The data for the tire above is on page 22 of this manual:
http://www.michelintruck.com/assets/...k_Sept2011.pdf
I just confirmed with Michelin using their online chat service that this 2011 edition is still current and that the M/S tire listed has the same load/inflation,eats as the M/S2 that I have on my trailer.
At 80 psi, the table says for single tire configuration (that's one tire on each end of the axle... which is what we have) the load rating is 5,360 pounds per axle. So, our two axle 25' trailers with those tires at 80 psi are set up for 10,720 pounds of load. I weigh my rig to tune the WD hitch and I know that when loaded for travel the trailer tips the scales at 6,600-6,800 pounds. However, some of that weight is tongue weight which is carried by the truck. Some of that tongue weight is then shifted back to the trailer by the hitch. The actual measurement of weight carried by my trailer's axles was 6,020 pounds at last measurement.
So, at 80 psi, my tires are inflated to carry 178% of their actual load (your figures may vary). That amount of over inflation should appear as excessive wear in the center of the tread pattern. In my experience, with about 30,000 miles on the tires, I see no evidence of wear... certainly no abnormal wear. The only way I know the tires are wearing at all is that it's become easier to snap my x-chocks in place as the space between the tires decreases. Over inflated tires theoretically will also show reduced braking effectiveness since less of the tire's tread is in firm contact with the road surface. Again, I don't see any evidence of that in practice and have been down grades of up to 11% and ask a lot of the trailer's brakes.
Back to the Michelin load/inflation table... the minimum listed pressure for the tires I run is 35 psi which shows a load rating of 3,000 pounds per axle. That's pretty darn close to the actual static load per axle I measure ver the CAT scales. I would not run my tires that low despite the authority the load/inflation table gives me to do so for three reasons: 1. The load is not always balanced evenly between the axles... for example when hitting rough pavement or when going over sharp crests or dips in the road 2. I know from experience that my gas mileage drops with lower trailer tire pressure. Given the already poor fuel economy of the Tundra, I'd need to bring a letter from my bank to head out on our typical spring 6,000 mile trip with that little air in the trailer tires and 3. Lower inflation can lead to higher running temperatures in tires. Despite the load/inflation table data, I'd worry on long stretches of hot highway in summer (think I-70 across Kansas) with the tires that low.
So, as I said above, for me it's 70 psi (measured with tires cold) but I would not debate anyone who said something between 60 and 80 is right for them. Having looked up the technical data from Michelin I might not even debate someone who goes lower although I'd be curious to understand how their experience supports lower running pressure given the three points I noted.
Happy (and safe... and comfortable) camping!
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