Even if you put tires on your RV that have a higher load rating and thus a higher max pressure does not necessarily mean that you will want to put more air pressure in them than you do now.
The tire should be inflated to the pressure specified by the tire manufacturer for the load on the tire.
See this Goodyear chart: https://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
Note that even if the max pressure on the ST225 75R15 is 80 PSI the recommended tire pressure is related to the load on the tire. Have your fully loaded trailer weighed and inflate according to the chart for your tire brand.
Too little air and the tire will tend to run hot and could lead to tire failure. To much air you might wear the tire prematurely in the center. Too much air also makes the ride hard which will add to vibration damage to your trailer.
It is a balancing act you should work to get right.
This may not answer your question directly but if your load says you should load to 65 PSI with your new tires the rating on the wheel becomes moot.
PS: Often wheels have the max load and or the max pressure stamped on the back side of the wheel and sometimes on the inside of wheel. If it is inside you would need to dismount the tire to see it.