This is how I decipher how the tire standards and regulations are supposed to work.
Replacement tires, at a minimum, need to provide the load capacity - via inflation pressures - that the OE tires provided.
The FMVSS says - in part - that vehicle manufacturers MUST set your trailer’s GVWR and each GAWR. To that is added the responsibility to select a tire/rim fitment that is appropriate
for each GAWR and set the recommended tire inflation pressure (s) for the selected tires. That’s not an arbitrary statement. The standard directs the vehicle manufacturer to do that. It’s a vehicle manufacturers responsibility and once made becomes a minimum standard for that vehicle.
Although the same FMVSS’s are used for automotive and RV trailers they do specify differences and it’s in those differences where the confusion often starts. Automotive fitments are required to have reserve load capacities. RV trailer fitments do not. Therefore an additional measurement is provided for RV trailer fitments and it says that the RV trailer manufacturers’ published hitch/pin weight (and they must publish one) when added to the trailer’s total GAWR must
equal or exceed the trailer’s GVWR. That in reality allows the trailer manufacturer to fit two 1500# tires to a 3000# axle. Kick-in the ST tire manufacturers on the one hand saying their tires are good to go all day on maximum allowed tire inflation pressures and you get inferior fitments that are surely going to fail early. And, they only have the option to set the recommended tire inflation pressures to maximum.
Setting RV trailer tire inflation pressures to accommodate the load carried is a poor recommendation by whoever makes it. A 1 psi loss of inflation pressure will cause such tires to be overloaded. You can lose a single psi by hitting a curb or pothole or just dropping off the slab you were parked on all weekend. Sure, it’s uncommon but it can happen.
Look on your tire placard, certification label or in the owner’s manual and find the size of the OE tires. Determine how much load capacity they provided at their recommended inflation pressure. Your new tires need to provide that much load capacity. Determine the inflation pressure you’re going to set to equal or exceed what the OE tires provided and jot it down in your owner’s manual. Auxiliary tire placards are allowed and it’s recommended that you make one and display it adjacent to the original tire placard or certification label.
I’m providing the following quote for those that misunderstand the purpose of the FMVSS standards. They are for vehicle manufacturers but are often quoted as something we, the users owners have control over.
The National Highway Traffic Safety Administration has a legislative mandate under Title 49 of the United States Code, Chapter 301, Motor Vehicle Safety, to issue Federal Motor Vehicle Safety Standards (FMVSS) and Regulations to which manufacturers of motor vehicle and equipment items must conform