Lets call the owner "Tex. He wrote:
"What is confusing is that tire pressure charts don't apply here. This is an older trailer and I may be trading it soon, but say I were going to keep it and put 17.5" wheels on it. Would I inflate to the maximum of 150 psi or would I refer to a pressure chart?"
My reply: Tire charts are based on the ability of a single tire to carry a specific load at a specific inflation and speed on a test drum while running straight ahead. Most vehicles have four tires, one at each corner. A few like some one-ton pickups and vans might have dual tires but the load in dual application is reduced based on years or real life experience that indicates that if you run tires in a close side by side basis you cannot get the same life and durability as when those tires are run on opposite ends of the same axle.
Now multi-axle trailers have been around for a long time but again most of these vehicles run the tires at the reduced "dual" load and also are probably running at the max pressure the tire is rated for. The use of multi-axles on RVs presents a different problem. The loads are not reduced to the "dual" load and the pressure on most RVs is not maintained at the max for the tire. In fact most RV trailers are running one or more tires in an overloaded condition.
RV manufacturers produce many units that are significantly out of balance side to side as they leave the production line. Since cost cutting is a major objective for both the manufacturer and the RV buyer, larger tires are seldom provided and in some cases games are played by de-rating the GAWR to allow smaller (less expensive) tires to be used. I have even seen a case where the RV manufacturer got a letter from the tire importer claiming the tire was capable of carrying more load than was standard for that size. Of course this "special" information is not provided to the RV owner, who will probably buy a replacement tire that shows the correct size but is not rated to carry the same load.
The manufacturers do not take into consideration the side to side unbalance, as to do so would require them to provide larger (more expensive) tires. The other thing RV industry does not take into consideration is the forces to the tire structure due to running close axle spacing. Engineering analysis shows that when turning corners the forces trying to tear the tire apart can be over 20% higher in multi-axle applications than with tires at the corners of the vehicle.
The only options for the trailer owner are to up-size the tires (if there is room) or up-rate and increase inflation (if a higher Load Range tire and stronger wheels are available). Lacking the above being sure to run the tire at the inflation shown on the sidewall (i.e. max) will slightly decrease but not eliminate the overload forces.
No, you should not run 150 psi. I doubt you will find that is the rating for the 17.5 tires or rims. What you should do is consult the load inflation tables and after getting accurate loads on each individual tire run no less than the minimum inflation for the heaviest tire on each axle. If you want to be more realistic and possibly see better tire durability for your tires, I would certainly consider using the "dual" load to establish the minimum inflation. I would further run +10% pressure to avoid day to day temperature and pressure variations. I would also consider a TPMS mandatory.